Carbonates within a Pleistocene glaciomarine succession, Yakataga Formation, Middleton Island,...

Carbonates within a Pleistocene glaciomarine succession,Yakataga Formation, Middleton Island, Alaska

NOEL P. JAMES*, CAROLYN H. EYLES� , NICHOLAS EYLES� , ERIC E. HIATT§ andT. KURTIS KYSER**Queen’s University, Kingston, ON, Canada K7L 3N6 (E-mail: [email protected])�McMaster University, Hamilton, ON, Canada L8S 4K1�University of Toronto at Scarborough, Scarborough, ON, Canada M1C 1A4§University of Wisconsin-Oshkosh, Oshkosh, WI 54901, USA

Associate Editor: Tracy Frank

ABSTRACT

Uplifted during the 1964 Alaskan earthquake, extensive intertidal flats around

Middleton Island expose 1300 m of late Cenozoic (Early Pleistocene) Yakataga

Formation glaciomarine sediments. These outcrops provide a unique window

into outer shelf and upper slope strata that are otherwise buried within the south-

east Alaska continental shelf prism. The rocks consist of five principal facies in

descending order of thickness: (i) extensive pebbly mudstone diamictite con-

taining sparse marine fossils; (ii) proglacial submarine channel conglomerates;

(iii) burrowed mudstones with discrete dropstone layers; (iv) boulder pavements

whose upper surfaces are truncated, faceted and striated by ice; and (v) car-

bonates rich in molluscs, bryozoans and brachiopods. The carbonates are

decimetre scale in thickness, typically channellized conglomeratic event beds

interpreted as resedimented deposits on the palaeoshelf edge and upper slope.

Biogenic components originated in a moderately shallow (ca 80 m), relatively

sediment-free, mesotrophic, sub-photic setting. These components are a mixture

of parautochthonous large pectenids or smaller brachiopods with locally

important serpulid worm tubes and robust gastropods augmented by sand-size

bryozoan and echinoderm fragments. Ice-rafted debris is present throughout

these cold-water carbonates that are thought to have formed during glacial

periods of lowered sea-level that allowed coastal ice margins to advance near to

the shelf edge. Such carbonates were then stranded during subsequent sea-level

rise. Productivity was enabled by attenuation of terrigenous mud deposition

during these cold periods via reduced sedimentation together with active wave

and tidal-current winnowing near the ice front. Redeposition was the result of

intense storms and possibly tsunamis. These sub-arctic mixed siliciclastic-

carbonate sediments are an end-member of the Phanerozoic global carbonate

depositional realm whose skeletal attributes first appeared during late

Palaeozoic southern hemisphere deglaciation.

Keywords Alaska, biogenic, carbonate, glaciomarine, Pleistocene, tsunami.

INTRODUCTION

The relationship between limestones and glaci-gene sedimentary rocks has long been a conun-drum; how can deposits formed in seeminglydisparate tropical and sub-polar palaeoenviron-

ments be juxtaposed? This question is particu-larly relevant for those periods in geologicalhistory when glaciation was intense and wide-spread, particularly during the late Palaeozoicand Neoproterozoic. Interpretations have rangedfrom wide swings in climate and sea water

Sedimentology (2009) 56, 367–397 doi: 10.1111/j.1365-3091.2008.00973.x

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists 367

chemistry, to distinctly separate periods of depo-sition, to coeval sedimentation in cold-waterenvironments (Fairchild, 1993). Whereas therehas been an increased understanding of cool-water carbonate depositional environments inrecent years, there is still a perception thatsignificant carbonate deposition does not occurin cold-water, sub-polar to polar environments.

The late Neogene sediments of south-easternAlaska, although mostly marine diamictites, docontain thin carbonate units. These carbonatesare exceptionally well-preserved on MiddletonIsland, a small high at the continental shelf edgesome 100 km from coastal Alaska (Fig. 1). Theisland is being elevated at the rate of ca 1 cm year)1

(Plafker et al., 1994) because it lies above thesubducting accretionary sedimentary wedgebeneath the Alaskan continental shelf. Uplift isepisodic as the island was elevated ca 3Æ5 m duringthe Good Friday Earthquake of 1964 (Plafker,1965). This event exposed over 1Æ3 km of the upperpart of the Yakataga Formation on a series ofextensive rocky tidal flats. The Yakataga Formationis a glacially influenced succession ca 5 km thickthat outcrops on land as a 500 km long and 30 kmwide belt in the foothills of the Saint Elias Moun-tains and passes beneath the continental shelf.The Yakataga Formation on the island is domi-nated by marine diamictite that is punctuated bynumerous fossiliferous conglomeratic carbonateunits decimetres in thickness.

The Yakataga Formation is an important bridgeinto the older rock record because the biota itcontains is largely extant, yet elsewhere similardeposits are mostly buried beneath modern con-tinental margin sediments and sedimentary rocks.The purpose of this paper was to determine thepalaeoenvironmental conditions that led to for-mation and preservation of carbonates in a sub-polar, ice-proximal setting. The report builds onprevious detailed studies of the Yakataga Forma-tion on Middleton Island and: (i) describes indetail the carbonate units that are interbeddedwith marine glacigene sediments; (ii) interpretsthe origin and timing of such deposits largely onthe basis of the modern and late Pleistocene of theGulf of Alaska; and (iii) places the deposits in thecontext of similar older carbonates and highlightstheir utility for interpretation of the ancient rockrecord.

SETTING

Introduction

This part of Alaska is characterized by activetectonism, spectacular glacier-covered, high,coastal mountains and intense weather systems.Alluvium and glacier-covered lowlands forestedby spruce and hemlock are backed by rugged2000 m high foothills that pass upward into the

Fig. 1. Chart of the Gulf of Alaska showing the location of Middleton Island and bathymetry of the continentalmargin, major glaciers and important oceanographic currents.

368 N. P. James et al.

� 2008 The Authors. Journal compilation � 2008 International Association of Sedimentologists, Sedimentology, 56, 367–397

Chugach – St Elias Mountains that rise abruptlyto over 4000 m. The macrotidal shoreline is oneof deeply indented inlets, bays and fjords, tide-water glaciers, extensive outwash and narrow,high-energy, ocean-facing beaches.

Middleton Island is a small, isolated ca 25 km2

island near the continental shelf edge at about60� N in the northern Gulf of Alaska roughly100 km from land (Fig. 1). It is the largest ofseveral small islands and rocky shoals that lie ontop of Tarr Bank, a large, shallow continentalmargin plateau that is part of the MiddletonIsland Shelf (Plafker et al., 1994) seaward ofPrince William Sound and the Copper River Delta(Fig. 1). The shelf here is ca 100 km wide andoriented roughly north-east to south-west with alinear depression inboard and the steep wall ofthe Aleutian Trench outboard. The central Mid-dleton Island Shelf is an extensive, circular,shallow bank (< 50 metres water depth, (mwd))bounded by submarine valleys, with local shal-low portions reaching sea-level as Tarr Bank,unnamed shoals and Middleton Island.

The Yakataga Formation contains the largest andmost complete sedimentary record of Late Pleisto-cene glaciation in the world. Geological aspects ofthe Yakataga Formation, in general, and that part ofthe succession exposed on Middleton Island, inparticular, are documented by Miller (1953), Plaf-ker & Addicott (1976), Allison (1978), Armentrout(1983), Eyles (1987, 1988), Eyles & Lagoe (1989)and Lagoe et al. (1989). The age of the deposits onMiddleton Island is not resolved precisely but,on the basis of microbiota, macrobiota, palaeo-magnetics and correlation with Ocean DrillingProgram (ODP) stratigraphy (Lagoe et al., 1993),consensus suggests that they accumulated duringthe late Pliocene and early Pleistocene (sensu lato;Remane, 1997; Shackleton, 1997; Pillans & Naish,2004).

Geological setting

The central part of the Gulf of Alaska adjacent tothe Aleutian Trench (the area of this study) is azone of compressive deformation along which thePacific Plate and Yakutat Terrane is underthrust-ing the continental margin. This effect results inextreme seismicity and high-frequency, large-magnitude earthquakes that lead to periodic massmovement of sea floor sediment and doubtlessnumerous tsunamis.

The Shelf is underlain by ca 5 km of lateCenozoic rocks deformed into extensively faultedtight anticlines (Plafker et al., 1994). Tectonism

has caused uplift of the shelf edge and tilting ofthe shelf such that strata on Middleton Island dipca 28� north-west. This arch began rising duringthe Miocene (Zellers, 1995) and earthquake-related uplift continues to the present time(Plafker et al., 1994). The upper part of thissection (ca 1300 m) is shallow-marine, cold-waterYakataga Formation siliciclastics.

Climate

Compared with other glaciated areas today theGulf of Alaska has a relatively mild, cool-temper-ate climate. Seasonal movement of the summerNorth Pacific high-pressure cell and the winterAleutian low-pressure cell dominates weathersystems. Climate is cool-temperate maritime,wet but with comparatively mild temperatures,and average annual precipitation ca 3000 mm(potential evaporation < 500 mm). Relativelywarm air masses, having travelled many hun-dreds of kilometres across the Gulf of Alaska, arecooled and forced to rise thousands of metres atthe coastline. Winter storms commonly stagnateover the gulf, being blocked by the mountains anda temperature-induced pressure gradient (Powell& Molnia, 1989). Moist maritime air from thesestagnant lows results in heavy precipitation thatfeeds glaciers at high elevation. Wet-based gla-ciers with abundant meltwater dominate the area.The only sea ice forms locally in fjords, the openocean is ice free.

Whereas average monthly wind speeds are aslow as 6 cm sec)1 in summer and as high as11 cm sec)1 in winter, the Aleutian Low cangenerate wind speeds of 100 to 150 km h)1 duringwinter leading to violent sea and unpredictablecurrents.

Oceanographic setting

The shelf is storm dominated wherein waveheights exceeding 40 m define one of the highestwave-energy environments in the world. Stormwave base is > 200 mwd resulting in sedimentmovement throughout the neritic environment(Powell & Molnia, 1989).

Circulation is dominated by the anticlockwiseSubpolar Gyre that advects waters eastwardsacross the Pacific Ocean at ca 50� N and directsthe Alaskan Current northward up the coast ofNorth America and westward along the southernmargin of the Aleutian Chain (Fig. 1). Thus,comparatively warm (> 5Æ0 �C) and saline(> 35&) waters are moved poleward at the

Exposed Pleistocene glaciomarine carbonates, Gulf of Alaska 369


edge of the shelf. Flow is strongest in winter(30 cm sec)1) because of intensification of thewinter low-pressure cell, and weakest in summerwhen winds tend to oppose the flow (Tomczak &Godfrey, 1994). Glaciers (Copper River Valleydelta, Icy Bay embayment, Malispina Glacier andBering Glacier) release large volumes of glacialmeltwater into the Gulf resulting in nearshoresummer salinities as low as 25&.

A relatively fresh Coastal Current flows alongthe inner shelf until it is forced out into theAlaska Current by Kayak Island (Fig. 1). Highrainfall, combined with glacial meltwater, pro-duces large annual mean freshwater sediment-laden discharge into the gulf. West of KayakIsland, where Middleton Island is located, dis-charge is today dominated by the Copper Riverwherein turbid sediment plumes during theautumn (maximum discharge from land) aretransported toward the north-west to west andeventually deposited nearshore. Some surfacesuspended sediment, however, becomes en-trained in large gyres and is transported offshore.The strongest gyre is over Tarr Bank and trans-ports 3000 to 24 000 tons of sediment per dayoffshore during the summer when it is mostactive (Powell & Molnia, 1989).

This part of the Gulf of Alaska lies within theAlaska Downwelling Coastal Province (Long-hurst, 1998) wherein downwelling is maintainedby density distribution at the coast resultingfrom dilution of surface waters, wind stresscurl and the longshore wind-induced slope insea-level. The only weak upwelling occurs to thewest, off the Alaska Peninsula near KodiakIsland, well to the west of Middleton Bank.Primary productivity is light limited in winterbecause of heavy overcast and nutrient-limitedin summer because of high surface freshwaterrun-off.

Sedimentation

Today glacier ice is grounded onshore and icebergtransport onto the shelf is minor because noglaciers extend beyond the littoral zone. Well-stratified glacial-marine sandy mud and mudfrom fluvial–glacial discharge have covered theinner and middle parts of the continental shelfduring the Holocene at a rate of ca 4Æ0 cm year)1

(Anderson & Molnia, 1989). It extends across theshelf as a seaward-thinning wedge and covers thearea south of Tarr Bank and north of MiddletonIsland indicating that offshore transport islimited.

Tertiary and Pleistocene stratified rocks formthe sea floor on Tarr bank, on the MiddletonIsland Shelf. Tarr Bank is covered with coarsebioclastic material, (Sharma, 1979; Powell &Molnia, 1989) but little information is available.The coarse material either is relict from the lastglaciation or, in the case of the carbonate, is beingproduced in place because coarse material is nottransported to offshore banks today (Sharma,1979; Powell & Molnia, 1989). If fine material isdeposited from suspension, it is resuspended bywave action and winnowed from the bank leavingthe outer banks as regions of siliciclastic non-deposition.

LITHOFACIES AND MACROFOSSILS OFTHE YAKATAGA FORMATION

Lithofacies

The stratigraphic succession on Middleton Islandis formed by six sedimentary lithofacies. Attri-butes of facies associated with the carbonates areoutlined in Table 1, positioned in Figs 2 and 3,and summarized below. The information pre-sented here is a synthesis of observations pre-sented in this paper, and those documented byEyles (1987, 1988), Eyles & Lagoe (1989), Lagoeet al. (1989) and Eyles et al. (1992). Specificattributes of the carbonates are described in thesubsequent section.

Even though the island is only 8 km long, faciesin the north are somewhat different to those in thesouth. Eyles & Lagoe (1989) interpret this dispo-sition to represent an inboard–outboard polarityand different water depths. In general terms, thesuccession begins with ca 320 m of channellizedsediment gravity flow conglomerates and turbi-dite sandstones (Fig. 3). Conglomerates are con-spicuously thicker in the south, whereas in thenorth coeval sediments are pebbly mud diamictsand gravel layers. The middle part from ca 320 to620 m is fossiliferous diamict with numerousboulder pavements cropping out in the north, anda few fossiliferous layers. The upper ca 600 m iscomposed of marine diamictite and numerouscalcareous conglomerates (the topic of this paper)with mudstones and boulder pavements at thetop.

Facies 1 – Channellized sandstone andconglomerateThese chaotic or graded sediments are interpretedas a submarine channel sequence deposited on



Table

1.

Yakata

ga

Form

ati

on

Lit

hofa

cie

son

Mid

dle

ton

Isla

nd

Facie

sC

om

posi

tion

Th

ickn

ess

Sed

imen

tary

stru

ctu

res

Macro

bio

taM

icro

bio

taIc

hn

ofo

ssil

ass

em

bla

ge

1.

Ch

an

nell

ized

san

ds

an

dgra

vels

Ch

aoti

cgra

vel-

dia

mic

t,m

ass

ive

an

dgra

ded

gra

vels

&sa

nd

s,m

ass

ive

dia

mic

t

Up

to200

mIn

vers

ean

dn

orm

al

gra

din

gP

ebble

imbri

cati

on

Ech

inoid

sB

en

thic

fora

min

ifera

(Elp

hid

ium

excavatu

mf.

cla

vatu

m)

VI

Dip

locra

teri

on

,P

ale

op

hycu

s,T

eic

hic

hn

us

2.

Bu

rrow

ed

mu

dst

on

eF

ine

san

dan

dm

ud

,sc

att

ere

dcla

sts

Local

bou

lder

layers

(dro

pst

on

es)

Up

to100

mF

ine

lam

inati

on

sB

iotu

rbati

on

Gast

rop

od

sB

en

thic

fora

min

ifera

(Ep

isto

min

ell

asp

.,U

vig

eri

na

sp.)

Pla

nkto

nic

fora

min

ifera

(Glo

big

eri

na

bu

lloid

es,

Neoglo

boqu

ad

rin

ap

ach

yd

erm

a)

IX Th

ala

ssin

oid

es,

Teic

hic

hn

us,

Dip

locra

teri

on

,P

ale

op

hycu

s,Z

oop

hycos,

Pla

noli

tes,

Are

nic

oli

tes,

Ast

ero

ma

3.

Mari

ne

Dia

mic

tite

Poly

mic

t,p

oorl

yso

rted

pebbly

mu

dst

on

ew

ith

stri

ate

dfl

oati

ng

cobble

s&

bou

lders

(dro

pst

on

es)

Up

to65

mM

ass

ive

Local

inte

rfere

nce

rip

ple

s&

hu

mm

ocky

–sw

ale

ycro

ss-l

am

inati

on

sL

ocal

defo

rmati

on

du

eto

iceberg

plo

ugh

ing,

slu

mp

ing

Gast

rop

od

s,se

rpu

lid

sB

en

thic

fora

min

ifera

(Elp

hid

ium

sp.)

VII

Teic

hn

ich

nu

s,D

iop

atr

ich

nu

s,R

hiz

ocora

lliu

m,

Zoop

hycos

4.

Bou

lder

pavem

en

tL

ayers

of

bou

lders

wit

hfa

cete

dan

dst

riate

du

pp

er

surf

aces

Up

to1

mL

ocal

defo

rmati

on

of

dia

mic

tabove

&belo

w

VII

Teic

hn

ich

nu

s,D

iop

atr

ich

nu

s,R

hiz

ocora

lliu

m,

Zoop

hycos

5.

Pla

nar

coqu

inas

Biv

alv

es

ina

calc

are

ou

ssa

nd

ston

eor

mu

dst

on

em

atr

ixw

ith

nu

mero

us

stri

ate

dbou

lders

Up

to1

mM

ass

ive

togra

ded

bed

s,w

ave

rip

ple

sP

ecte

ns;

serp

uli

ds;

&biv

alv

e,

bry

ozoan

,ech

inoid

,barn

acle

,gast

rop

od

fragm

en

ts

Ben

thic

fora

min

ifera

(Cass

idu

lin

acali

forn

ica)

VII

IG

ast

roch

aen

oli

tes,

Th

ala

ssin

oid

es

6.

Ch

an

nell

ized

foss

ilif

ero

us

con

glo

mera

tes

Mu

ds,

debri

tes,

gra

ded

san

dtu

rbid

ites

Up

to3

mM

ass

ive

togra

ded

bed

s,la

min

ati

on

sB

rach

iop

od

s,se

rpu

lid

s,p

ecte

ns,

oth

er

biv

alv

es

Ben

thic

fora

min

ifera

(Cass

idu

lin

acali

forn

ica)

VII

IG

ast

roch

aen

oli

tes,

Th

ala

ssin

oid

es

Ich

nofo

ssil

ass

em

bla

ges

–E

yle

set

al.

(1992).



the upper continental slope in water depths of asmuch as 200 m (Eyles, 1987).

Facies 2 – Structureless and burrowed mud-stoneThese mudstones are envisaged as interglacialdeposits that accumulated in deep water. Glaciersare thought to have been sequestered inboard,sea-level was high, and there was prolific out-wash, the fine-grained portion of which wastransported out onto the shelf.

Facies 3 – Marine diamictiteThe Yakataga Formation on Middleton Island isdominated by this facies of fossiliferous diamic-tites (Fig. 4) thought to have accumulated byrainout onto the sea floor of fine-grained sedimentfrom meltwater plumes and ice-rafted clasts.Water depths are interpreted as neritic (Lagoeet al., 1989; Eyles et al., 1992).

Facies 4 – Boulder pavementBoulder pavements are planar horizons of largeclasts whose faceted and striated upper surfacesshow a consistent direction of ice movementoffshore to the south; they rest directly onbioturbated diamictite. Such lag deposits are

interpreted as recording winnowing of under-lying diamictite during low stands of sea-levelwith surface planation and striations developingbelow partially floating ice lobes thatextended to near the continental shelf edge(Eyles, 1988).

Facies 5 and 6 – CarbonatesThese metre-thick deposits (Kaye et al., 1985;Eyles & Lagoe, 1989) range from coarse bivalve-rich rudstones to fossiliferous conglomerateswith a siliciclastic mud or sand matrix. Follow-ing the terminology of Eyles & Lagoe (1989), thecarbonates are of two different types: (i) planarcoquinas; and (ii) channellized fossiliferousconglomerates. The term coquina is usedherein as a general epithet to encompass a widevariety of lithologies, all of which are rich invalved shelly fossils, particularly bivalves andbrachiopods. Likewise, the term calcarenite isused to describe a sediment with both silici-clastic and carbonate components, mostly ofsand size. Many calcarenites are channellizedand record a complex history of filling; they areinterpreted as having accumulated in relativelyshallow-water, neritic (20 to 80 mwd) environ-ments.

Fig. 2. Middleton Island showing the disposition of the Yakataga Formation exposed on the recently upliftedintertidal platform, the location of different facies, boulder pavements, planar coquinas and calcareous conglo-merates as well as the locations of stratigraphic sections in Fig. 3 (from Eyles & Lagoe 1989).



Fig. 3. Stratigraphy of the Yakataga Formation as exposed on Middleton Island (modified from Eyles & Lagoe 1989).



Macrofossils

Macrofossils in the Yakataga Formation are dom-inated by molluscs, especially bivalves (Clarke,1932; MacNeil, 1967; Addicott et al., 1971; Kaye,

1984). The most abundant macrofossil is theextinct pecten Chlamys pseudislandica plafkeri.Other less numerous pectens include Chlamyshanaishiensis amchitkana and Chlamys (Leochla-mys) tugidakensis. Additional conspicuous

A B

C D

E F

Fig. 4. (A) Areal view of diamictite and burrowed mudstone exposed at low tide along the west coast of MiddletonIsland (exposed tidal flats ca 1 km wide). (B) Thin layers of dropstones and isolated dropstones in burrowed mudstone,in the intertidal zone illustrated in A (hammer for scale – circled). (C) Diamictite with scattered large dropstones.(D) Close view of diamict with isolated serpulid worm cluster (4 cm of hammer tip for scale at left), inset photo-micrograph of benthic foraminifera from diamict. (E) Gastropods in diamict (hammer for scale at right). (F) Interferenceripple marks in diamict below Q2. Scale divisions 2 cm. Blade of hammer used for scale is 15 cm long.



bivalves are the neritic epibenthic mussels Myti-lus sp. and Modiolus sp. together with theburrowing myaid Mya truncata. Less numerousforms are the macractid Spisula ramonensis, thetellin Macoma nasuta and the cartitid Clinocar-dium sp. The gastropod Neptunia lirata floats indiamictites, whereas Cryptonautica aff. C. clau-sa., Cancellaria (Crawfordina) alaskensis andPriscofsus aff. P. hannibali occur in the coquinas.

Serpulid worm tubes are conspicuous in allcarbonates. Echinoids are mainly regular, epifau-nal types, brachiopods are all terebratulids, andbryozoans are both erect and encrusting forms.Barnacles are usually present as isolated platesand locally as whole skeletons.

SEDIMENTOLOGY OF THE COQUINASAND FOSSILIFEROUS CONGLOMERATES

Type 1 – Planar coquinas

These are the most calcareous of the carbonatesand form metre-scale, subtabular beds with abroad, shallow channel geometry (Figs 5 to 8).Units are designated by an alphanumeric code(e.g. Q1) on Figs 2 and 3; their attributes areenumerated on Table 2, while Figs 5 and 6 aresketches of their main attributes. These unitshave erosive bases, sharp or gradational tops(Fig. 7A), and internally comprise multiple eventbeds (Fig. 7F). Compositions range from Chlamys

Fig. 5. A field sketch illustrating the main attributes of planar coquina Q2; see Figs 2 and 3 for location. Erosionaltop of diamict is an omission surface.



coquinas (Fig. 7B to D) to poorly sorted lithicclast conglomerates with numerous to sparsemacrofossils. The matrix ranges from a muddyto grainy sandstone (wacke to arenite) withvariable bioclasts to a limestone (skeletal grain-stone). All units contain conspicuous outsizedclasts (Fig. 7B) up to 0Æ5 m in diameter. Thesecoquinas are restricted to strata on the northernend of the island.

Although five layers are present, the units Q1and Q2 (Fig. 3) are the best exposed and mostinstructive. Unfortunately, modern sedimenta-tion of intertidal sands and muds since the 1964earthquake has all but obscured two of thecoquinas figured by Eyles & Lagoe (1989).

Each coquina is formed of multiple units(Figs 5 and 6), every one of which compriseserosion (channellization), sedimentation (fillingof channels) and deposit reworking (shell imbri-cation). As such, each coquina is not a singleevent bed but a series of superimposed or stackedevent beds, with the implication that togethersuch beds may span a significant period of time.These attributes also demand that sediment wastransported from a source area; with the caveatthat such an area may not have been far away.

The biota, particularly serpulids and benthicforaminifera, supports the notion of non-deposi-tion periods between depositional events

(see below). Finally, there is generally no silici-clastic mud between event beds, indicating that itwas either eroded prior to deposition of theoverlying unit or terrigenous sedimentation wasarrested. The benthic foraminiferal assemblagesupports this interpretation (see below).

There are, however, units in Q2 with conjoinedpecten valves in muddy sand matrix (Fig. 5).Such conjoined pectens that exhibit little, if any,breakage (Fig. 8C) and all manner of orientationsin a muddy sand with scattered cobbles implytransport and deposition as a sediment gravityflow. Alternatively, such bivalves may be largelyin situ and have undergone post-mortem distur-bance by burrowing infauna. Thus, even thoughthe commeasure is not horizontal, the joinedshells may be close to in place. If these shells are,however, allochthonous, they must have beentransported with little particle interaction as adebris flow wherein fluid pore pressure was thedispersal mechanism and the deposit ‘froze’ inplace when movement ceased (Middleton &Southard, 1984); thus, they may be, in a strati-graphic sense, ‘condensed horizons’.

All coquinas contain outsized clasts (cobbleand boulders up to 0Æ5 m in diameter), most ofwhich are textured with glacial striations. Incoquinas Q1 and Q2, however, these bouldersrarely extend below the beds into underlying

Fig. 6. A field sketch illustrating the main attributes of planar coquina Q1; see Figs 2 and 3 for location. Erosionaltop of mudstone is an omission surface.



sediment (Fig. 7B), are enclosed by shell concen-trations that wrap around the clasts and do notobviously crush delicate shells, indicating thatthey were not dropstones into the coquinas aspresently constituted. Thus, these clasts origi-nated in the source area (where they were prob-

ably dropstones) and were transported, alongwith the shells, as part of the deposit, to thepresent site of deposition.

Sand-size carbonate particles in the coquinascontain abundant bioclasts. Whereas most wereformed by disintegration of skeletons (branching

A B

C D

E F

Fig. 7. (A) Planar coquina Q2 resting on muddy diamict (hammer for scale). (B) Coquina Q2 with numerous pectens(around scale) and outsized clast (scale divisions 2 cm). (C) Surface of coquina Q2 with numerous pectens in allorientations (scale divisions 2 cm). (D) Coquina Q2 surface with numerous nested pectens. (E) Vertical sectionthrough graded bed (Q1) with pebbles and cobbles at base and some pectens at top, matrix is grainstone (Fig. 10E). (F)Coquina Q1 made up of three units (labelled). Blade of hammer used for scale is 15 cm long.



bryozoans, echinoderms), or were originallyindividual grains (foraminifera), the serpulidsare typically angular pieces, indicating singleevent fragmentation. The sand-size molluscs are

different, generally rounded, implying residencein an environment of high energy (cf. Hoskin &Nelson, 1969) and microbored, indicating possi-ble residence in the photic zone.

A B

C D

E F

Fig. 8. (A) Surface of coquina Q2, arrows point to crests of large wave ripples. Person for scale is approximately 2 mtall. (B) Base of coquina Q1 with numerous nested pectin shells. Blade of hammer used for scale is 15 cm long.(C) Coquina Q2 with numerous conjoined pectin valves in pebbly mud. (D) Boulder encased in an encrustingbryozoan that is in turn encrusted by a serpulid worm tube (centimetre scale), inset is another cobble encrusted by abryozoan. (E) Close view of pectin shell (left) Celleporaria sp. bryozoan (arrow) and serpulid worm tubes in coquinaQ1. (F) Cross-section of two pectin shells each of which is encrusted by serpulid worm tubes, coquina Q2.



Table

2.A

ttri

bu

tes

of

coqu

inas

an

dfo

ssil

ifero

us

con

glo

mera

tes

Un

itE

nclo

sin

gfa

cie

sD

esc

rip

tion

Matr

ixF

ora

min

ifera

lbio

facie

s

Coqu

ina

Q2

•B

elo

w–

dia

mic

tite

•A

bove

–bu

rrow

ed

mu

dst

on

e

•1

to2

mth

ick,

ch

an

nell

ized

•L

ow

er

1/2

=bou

lder

cgl.

&p

ecte

nfl

st.;

foss

ils

=p

ecte

ns

&se

rpu

lid

s;p

ecte

ns

nest

ed

(con

cave-d

ow

nat

base

all

ori

en

tati

on

sin

mid

dle

,con

cave-u

pat

top

);fl

stw

ith

con

join

ed

pecte

ns

•U

pp

er

1/2

=se

rpu

lid

-ric

hcobble

cgl.

;se

rpu

lid

sfr

ee

&att

ach

ed

;bry

ozoan

s(i

nclu

din

gC

ell

ep

ora

ria),

gast

rop

od

s,p

ecte

ns;

overl

ain

by

an

dgra

des

into

serp

uli

dfl

st.

&rd

st

•P

ackst

on

e;

matr

ix=

mic

rosp

ar

or

sili

cic

last

icm

ud

;m

inor

inte

rpart

icle

solu

tion

seam

san

dd

rusy

cem

en

t3

to5

lm

thic

k•

Poorl

yso

rted

calc

are

nit

e(e

qu

al

am

ou

nts

of

carb

on

ate

&si

licic

last

icp

art

icle

s)•

Poly

mic

tte

rrig

en

ou

scla

stic

fin

esa

nd

togra

nu

les,

an

gu

lar

tow

ell

-rou

nd

ed

•C

arb

on

ate

s=

1/3

serp

uli

ds,

2/3

bry

ozoan

s,ech

inoid

s&

pecte

ns

•B

ryozoan

s=

small

rigid

,d

eli

cate

bra

nch

ing

ch

eil

ost

om

es

•N

on

efo

rth

isu

nit

bu

tu

nit

above

con

tain

sE

.excavatu

mf.

cavatu

min

dia

mic

tite

sbelo

wan

dU

vig

eri

na

jun

cea

inm

ud

ston

es

above

•Im

pli

es

rap

idd

eep

en

ing

foll

ow

ing

dep

osi

tion

Coqu

ina

Q1

Belo

w=

mu

dst

on

e;

dir

ectl

ybelo

ware

coars

ed

iam

icti

te,

thin

sst.

wit

hin

terf

ere

nce

rip

ple

s,an

dd

rop

ston

es,

or

HC

San

dS

CS

,or

ch

aoti

call

yd

efo

rmed

an

dfr

agm

en

ted

sst.

Above

=sp

ars

ed

iam

icti

te;

bou

lder

pavem

en

t50

mabove

•L

en

ticu

lar,

ch

an

nell

ized

pecte

n-r

ich

lith

icp

ebble

cgl.

;ou

tsiz

ed

bou

lders

or

pecte

ncoqu

ina

&cro

ss-b

ed

ded

gn

st.-

cgl.

;w

ood

an

dazooxan

thell

ate

cora

ls.

•B

asa

lu

nit

=li

thic

pebble

cgl.

,bou

lders

up

to40

cm

dia

mete

r,p

ecte

ns

nest

ed

–vert

ical

or

con

vex-d

ow

nori

en

tati

on

•M

idd

leu

nit

=cro

ss-b

ed

ded

pecti

ngn

st.-

cgl.

;n

um

ero

us

ou

tsiz

ed

cla

sts;

sub-a

qu

eou

sd

un

es

(a=

20

cm

,l

=2

to3

m,

1Æ5

cm

thic

kfo

rese

ts),

flow

ton

ort

h,

on

-sh

elf

.•

Up

per

un

it=

pecti

nrd

st.

Main

lyin

trou

gh

s;p

ecte

ns

nest

ed

con

cave-u

p

•G

rain

ston

e;

10%

to30%

sili

cic

last

iccom

pon

en

ts;

ph

ysi

cal

com

pacti

on

=p

art

icle

bre

akage;

gra

ins

inst

ylo

con

tact;

insi

gn

ifican

td

rusy

calc

ite

cem

en

tas

3to

5lm

thic

kfr

inges

•D

eli

cate

bra

nch

ing

bry

ozoan

s=

1/3

gra

ins;

oth

ers

=se

rpu

lid

s,p

ecte

nfr

agm

en

ts,

ech

inoid

s•

Bry

ozoan

s=

low

div

ers

ity,

man

yzooecia

fill

ed

wit

hp

hosp

hate

•M

oll

usc

sm

icro

bore

d•

Con

spic

uou

sfo

ram

inif

er

test

s(e

specia

lly

Elp

hid

ium

)an

dbra

ch

iop

od

fragm

en

ts

•S

ed

imen

tsabove

an

dbelo

w=

E.

cavatu

mf.

excavatu

mbio

facie

s•

Coqu

ina

=C

.cali

forn

ica

bio

facie

s



Table

2.C

on

tin

ued

Un

itE

nclo

sin

gfa

cie

sD

esc

rip

tion

Matr

ixF

ora

min

ifera

lbio

facie

s

Foss

ilif

ero

us

con

glo

mera

te•

Lit

hic

are

nit

etu

rbid

ites

&bu

rrow

ed

mu

dst

on

es

•L

ocall

yh

igh

lyin

cli

ned

,fo

lded

,ch

aoti

call

yd

efo

rmed

•Is

ola

ted

or

nest

ed

ch

an

nels

;1

to3

mw

ide,

10

to20

cm

thic

kfi

lled

wit

hm

ud

dy

san

dy,

spars

e-r

ich

,cla

stto

matr

ix-s

up

port

ed

lith

iccla

stcgl.

;vari

able

macro

foss

ils

•S

tria

ted

lith

iccla

sts

or

defo

rmed

sst.

1to

2m

insi

ze,

like

surr

ou

nd

ing

bed

s•

Pecte

ns

nest

ed

&con

cave-u

p;

dis

cre

tecm

-th

ick

gast

rop

od

&bra

ch

iop

od

layers

•T

op

sof

som

eu

nit

scru

dely

stra

tifi

ed

pecti

n-b

rach

iop

od

-gast

rop

od

gn

st•

Local

m-s

ize

bou

lders

=d

rop

ston

es

•M

ore

bra

ch

iop

od

san

dM

od

iolu

sth

an

coqu

inas

•B

rach

iop

od

s>

pecte

ns

>oth

er

biv

alv

es;

afe

wgast

rop

od

s,en

cru

stin

gbry

ozoan

son

pro

tru

din

gp

ebble

s,se

rpu

lid

s•

Bra

ch

iop

od

s=

con

join

ed

valv

es,

sin

gle

valv

es,

nest

ed

shell

s

•M

ed

ium

-coars

egra

ined

lith

icare

nit

es

wit

h10%

to20%

bio

fragm

en

ts;

poorl

yso

rted

,cem

en

ted

by

mic

rocry

stall

ine

calc

ite

•B

iofr

agm

en

tsh

igh

lyvari

able

;ech

inoid

s=

10%

to70%

;ben

thic

fora

min

ifera

(esp

ecia

lly

Elp

hid

ium

sp.)

locall

y=

50%

;•

Bry

ozoan

s=

0%

to50%

;se

rpu

lid

s=

10%

Bry

ozoan

s=

ere

ct

rigid

deli

cate

bra

nch

ing

ch

eil

ost

om

es

&ro

bu

sth

eavil

ycalc

ified

form

s

•S

ed

imen

tsbelo

wtu

rbid

ites

con

tain

up

per

bath

yl-

ou

ter

neri

tic

ass

em

bla

ge

Sst

.,sa

nd

ston

e;

Cgl.

,con

glo

mera

te;

Fls

t.,

floats

ton

e;

Rd

st.,

rud

ston

e;

Gn

st,

gra

inst

on

e;

Pkst

.,p

ackst

on

e.



Trace fossils at the base and top of the coquinasare characteristic of the Glossofungites Ichno-facies (Pemberton & Frey, 1985). Sharp-walledburrows below penetrate diamicts or wave-rippled sandstones and are filled by fragmentedshells and benthic foraminifera indicating a firmsubstrate that was open at the time of coquinaemplacement, a hiatal surface. Upper surfaces arepenetrated by bivalves that formed cementedcylindrical burrows (e.g. Conichnus); their pres-ervation in burrows indicates that they weresmothered by the sudden influx of fine sediment.

Type 2 – Channellized fossiliferousconglomerates

These rocks (Figs 9 and 10) are individual chan-nels (Fig. 10A) cut into diamict and filled withconglomerates that are generally less calcareous,less persistent and thinner than those describedabove. Units are designated on Figs 2 and 3 by analphabetic code (e.g. QB; their attributes are out-lined in Table 2, while unit QD is sketched inFig. 9). Such conglomerates occur mostly on thesouth end of the island, but are also present inlower parts of the section on the north side. Ben-

thic foraminifera in sediments above and belowindicate an outer neritic to upper bathyl setting,deeper than the planar coquinas. The biota is alsosomewhat different and although containingnumerous Chlamys, is dominated by brachiopods,mussels and serpulid worm tubes.

INTERPRETATION OF COQUINAS ANDFOSSILIFEROUS CONGLOMERATES

Both the coquinas and fossiliferous conglo-merates are allochthonous in that they are variablymodified event beds. Thus, any interpretationmust address: (i) the nature of the carbonatesource (factory) environment; (ii) the mechanismsof redeposition; and (iii) how these attributes canbe resolved in a highly fluctuating glaciomarinepalaeoenvironment.

Original depositional environment (the sourcearea)

General macrofossil distributionYakataga Formation molluscs along the mainlandcoast were monographed by Clarke (1932) with

Fig. 9. A field sketch illustrating the main attributes of calcareous conglomerate QD; see Figs 2 and 3 for location.



the taxonomy updated by Addicott et al. (1971).Plafker & Addicott (1976) conclude that themajority of Yakataga Formation molluscs grewin waters < 100 m deep, with most living between20 and 60 m.

Bivalves

Scallops. The most abundant bivalve in Middle-ton Island strata is the extinct scallop C. pseudi-slandica plafkeri (Figs 7C, 7D and 8C). Chlamys

A B

C

D

E F

Fig. 10. (A) Numerous serpulids and bivalve at top of Q2, hammer tip for scale. (B) Clast encrusted with numerousserpulids, Q2. (C) Channellized muddy conglomerate (arrows point to the base of the channel) (hammer for scale).(D) Surface of brachiopod-rich muddy conglomerate, inset cleaned brachiopod – centimetre scale. (E) Thin sectionphotomicrograph, plane polarized light of grainstone in Fig. 7E, composed of bryozoans (Myriapora sp.), microboredbivalve fragments and pieces of serpulid tubes. (F) Fracture in gastropod shell rimmed by fibrous calcite cement.Blade of hammer used for scale is 15 cm long.



pseudislandica, the Bering scallop, is foundtoday from California to the Gulf of Alaska. It isclosely related to the Atlantic form Chlamysislandica and in the past these animals wereconsidered the same (Waller, 1991). Other lessnumerous and extinct pectens include the closelyrelated C. hanaishiensis amchitkana and C. (Leo-chlamys) tugidakensis.

Chlamys pseudislandica and C. islandica livein areas of strong current movement in thenorthern boreal or subarctic transition zone.Although favouring zones of active current move-ment, particularly tidal currents (Arsenault &Himmelman, 1996), these bivalves are inhibitedby very high rates of current flow (Brand, 2006);they are byssally attached throughout life, but candetach and swim if threatened by predators,especially asteroids. The animal lives in watersfrom )1Æ5 to 9 �C with the largest concentrationsin waters 10 to 100 m deep. Preferred habitats arecoarse, hard substrates, of sand, gravel and shells.These bivalves can live for 20 years, achieve sizesof 60 to 110 mm in length and grow with bottomdensities of 10 to 70 m)2. The animals do not liketurbid waters, thrive in fully marine waters, butcannot tolerate salinities of less than ca 28 &.One of the common causes of mass mortalityof shallow-water high-latitude scallops is thehyposaline lens formed under sea ice and seaice melting during the late summer (Stockton,1984). The organisms live with the slightlyinflated right valve down, the commissure hori-zontal, and the upper valve slightly agape, withthe umbo generally into the current (Brand, 2006;Lauzier & Bourne, 2006; Strand & Parsons, 2006).

These attributes confirm that the environmentof growth was about the same temperature as theregion today, of normal salinity (although notruling out periods of freshwater formation), rela-tively active water movement (probably tidal), arocky to boulder to shell-covered sea floor inrelatively clear waters roughly 20 to 100 m deep.These conclusions match those of Allison (1978).None of the scallops are in growth position(Figs 7C, 7D and 8B), even those conjoinedbecause they float at all orientations in muddyunits (Fig. 8C).

Mussels. The two mussels are Modiolus sp. andMytilis sp. both of which live today in the shallowphotic zone (to depths of ca 50 m) and can beintertidal (Pojeta, 1987). Modiolus is shallowinfaunal to marginally epifaunal with a weakbyssal attachment. The less numerous Mytilis isan epifaunal form that typically is attached to

cobbles and pebbles in zones of relatively highenergy (Stanley, 1970) and laminoid brown algae,e.g. Laminaria.

Other bivalves. The other conspicuous bivalve,M. truncata, is a relatively deep burrowing animalthat, although common in shallow waters, alsogrows in large numbers to depths of 100 m ormore on Spitzberbgen bank (Henrich et al., 1997).All bivalves observed have relatively delicateshells that are neither broken nor abraded, sug-gesting minor transport or reworking. There are afew rare oysters encrusting on gastropods infossiliferous channellized boulder conglomerates.

Gastropods (Fig. 4E)The most numerous gastropods are Neptunealyrata, Natica (Cryptonatica) clausa, C. (Crawfor-dina) alaskensis and Pricifusus hannibali. All arepredatory snails (Abbott, 1974). The whelkN. lyrata occurs isolated in massive diamicts. Itis puzzling why this snail should be in sedimentwith few other organisms except if it were frozento the ice bed in shallower water and eventuallyfreed by ice melting and dropped on the open seafloor. The whelks C. alaskensis and P. hannibali,and the moon shell C. clausa, are found predom-inantly in planar coquinas. All gastropods havewide depth ranges from intertidal to over 500 m(Keen & Coan, 1974).

Brachiopods (Fig. 10B)Most of the terebratulids are pedically attachedforms and as such require a hard substrate, in thiscase lithoclasts or mollusc shells. None areactually found attached. There are at least twospecies of large terebratulids of the genus Laqu-eous present. Details of internal structure are notwell-preserved, prohibiting specific identifica-tion.

Serpulid wormsSerpulid worms (Figs 8E, F and 10C, D) are foundin abundance throughout, are a major carbonatecomponent and their distribution has importantimplications. Serpulid worms occur:

1 Complete and unabraded attached to theinner and outer surfaces of scallops and pebblesin the coquina matrix sediment, indicatingencrustation before burial. In the case of litho-clasts this implies either erosion of the seafloor below or transport from an environmentwhere pebble encrustation was common. Theseattributes together indicate that some Chlamys



were dead on the sea floor before transport, andthat transport was minor.

2 On the surface of boulders protruding abovethe coquina, yet covered with overlying muddysediment, pointing to periods of non-sedimenta-tion following clast deposition. Such a relation-ship indicates that the coquina units were zonesof non-deposition following emplacement, aninterpretation supported by the presence ofthe benthic foraminifer Cassidulina californica(Lagoe et al., 1989).

3 As prolific encrusters and particles at the topof some units, implying prolonged periods ofencrustation and slight reworking (because theyare not fragmented).

4 As numerous pieces in the sand-size frac-tion, indicating that the source was at least para-utochthonous; the fact that they are rarelyrounded and generally angular implies biofrag-mentation.

5 As isolated clusters and encrusting isolatedpebbles in diamictites, implying that they hadfallen into the muddy sediment. This occur-rence is troublesome in that they could haveonly come from the sea floor and been trans-ported by ice. The most likely explanation, asthey are unabraded, is that they were frozeninto the base of ice, plucked from the sea floorwhen the ice lifted off, transported as frozenmaterial, and deposited seaward as fallout whenthis ice melted.

BryozoansBryozoans are recurring and conspicuous sand-sized particles in the grainy sediment. The mostabundant grains are pieces of the erect-rigid,bifurcating branched form Myriapora sp.(Fig. 10E). This bryozoan is by far the mostabundant type living and forming bryozoan-richsediment today between water depths of 40 and100 m on top of the Kodiak Shelf (Fig. 1) to thewest of Middleton Island Bank (Cuffey & Turner,1987).

Bryozoans also encrust the surfaces of litho-clasts (Fig. 8D) and pectens in coquinas andconglomerates, especially the insides of valvesand clasts protruding from the tops of units. Themost recurring form is Smittia sp., mostly onpebbles and boulders; they are, in turn, locallyencrusted by serpulids implying prolongedexposure on the sea floor prior to any transport.The other conspicuous bryozoan is the robustarborescent form Celleporaria sp. (Fig. 8E) withnumerous examples exhibiting secondary

calcification, a trait of bryozoans growing inhigh-energy environments. The zooecia of allbryozoans commonly are filled with phosphatesuggesting nutrient-rich conditions. Bryozoansfeed on unarmoured plankton thus confirmingthe presence of adequate nutrients in near-surface waters and that shallow parts of thewater column were not overly turbid. Further-more, Celleporaria supports the notion of alargely mesotrophic water column (Hagemanet al., 2003).

BarnaclesBarnacles are rare, but present. Fragments ofbarnacles are found on some pebbles.

Fragmented biotaSand-sized components (Fig. 10E) within thematrix, in addition to serpulid worm tube frag-ments, are mostly bryozoans, echinoids and mol-luscs. Echinoids confirm that bottom waters weremainly of normal, open ocean salinity. The erect-rigid, branching bryozoans can disintegrate toform sand-size particles (cf. Bone & James, 1993);their overall unabraded nature, however, impliesminimal transport, suggesting that they arelargely autochthonous.

Most mollusc fragments are bivalves and,whereas some are clearly scallop pieces andothers are gastropods, most are of indeterminateaffinity. These pieces are rounded and micro-bored. Borings are straight to slightly curved 2to 4 lm diameter tubules that occasionally arebranched. Lack of swellings, little dichotomousbranching, no tapered tubules and their randomdistribution as opposed to being localized toorganic lamellae suggest that they are not fungalborings (cf. Golubic et al., 2005) but are cyano-bacterial (Jones, 1988; Golubic et al., 2000) andhence phototrophs. There are no other photo-trophs in the deposits. These attributes have twoimplications: (i) that the molluscs were fromanother, possibly shallower environment; and, ifso, (ii) that the coquinas were transported anddeposited below the euphotic zone.

Microfossils (Fig. 4D)The rocks contain numerous foraminifera, one ofthe most ubiquitous is Elphidium excavatumf. clavatum (Lagoe et al., 1989). This benthicspecies in modern and Late Quaternary Arcticseas is related to cold (< 1 �C) marine waters (Haldet al., 1994). More specifically, it is most abun-dant in turbid waters that are seasonally icecovered close to the terminus of glaciers, and in



shallow settings covered by low salinity waters(ca 33&), in areas of high sedimentation rates. Itis infaunal, living 6 to 10 cm below the sedimentsurface. In particular, it dominates the marineforaminiferal biotas in sediments proximal tocalving glaciers. On the shelf off Yakatak Bayand Icy Bay just south-east of Middleton IslandBank (Fig. 1), this marginal marine species liveson sandy substrates in the photic zone < 100 mdeep that are subject to moderate to highturbulence (Lagoe et al., 1989).

Lagoe et al. (1989) recognized four benthiccalcareous foraminiferal biofacies on MiddletonIsland and interpreted them relative to modernGulf of Alaska biofacies. The E. excavatumf. clavatum biofacies was inner neritic, ca 10 to80 m deep, photic and localized to sandy sub-strates. The Cassidulina norcrossi s.l. biofacieswas subphotic in outer neritic depths of ca 80 to150 m within muddy sediments. The Uvigerniajuncea biofacies was the deepest in outermostneritic to upper bathyl muddy environmentsca 150 to 250 m deep. The distinctive Cassidulinacalifornica biofacies generally was neritic butconfined to firm or hard substrates where therewas little or no net sediment accumulation.Temporal distribution of these biofacies confirmsrepeated bathymetric fluctuations in the order of100 m within the section on Middleton Island.

Planktonic foraminifera from the deepest water,relatively rare, U. juncea biofacies suggest oceanwarming associated with such deepening andinterglacial relative sea-level rise.

The C. californica biofacies is localized tocoquinas and boulder pavements, confirming epi-sodic emplacement followed by sediment starva-tion. Most coquinas (such as Q1) are underlainand overlain by E. excavatum clavatum biofacies,verifying deposition within the inner neritic zone.One coquina (QB) while underlain by E. excava-tum clavatum biofacies is immediately overlainby U. juncea biofacies indicating a rapid rise inrelative sea-level immediately after deposition.

SynthesisThe overall biota points to an original deposi-tional environment with relatively clear, althoughpossibly turbid, cold (< 5 �C) bottom waters ofnormal salinity with good trophic resources(mesotrophic) and active water movement.Macrofossils and microfossils point to a relativelyshallow neritic sea floor < 100 m in depth andpossibly much shallower. The community iswell-structured with both sessile filter feedersand predators.

Lack of phototrophs or herbivores, however,implies subphotic water depths. The photic zonein sub-polar settings such as Alaska is dominatedby profuse phaeophytes, except where the kelpare grazed and so replaced by ubiquitous coral-line algae (Freiwald & Henrich, 1994). In shallowsubaqueous phaeophyte Laminaria forests (< 25mwd) on Spitzbergen Bank, for example, the areabetween holdfasts is covered with profuse barna-cle growth, that produces widespread Balanusfragment sand (Henrich et al., 1997). Barnaclesare scarce in Yakataga Formation carbonates.Encrusting corallines are numerous in the inter-tidal zone on Middleton Island today (this study)and elsewhere in rocky peritidal Gulf of Alaskaenvironments (Hoskin & Nelson, 1969, 1971).Thus, lack of any phototrophs and very shallow-water indicators in an area where they are presentimplies subphotic depths for the Yakatagasediment factory.

Sea floor illumination, however, is not only afunction of latitude and season, but water trans-parency and clarity can also be reduced dramati-cally by suspended sediment. The Gulf of Alaskais below the Arctic Circle and so winter darknesswas not a critical issue, although seasonal icecover and low sun angle may have been. Calcu-lations indicate that at least during the last glacialmaximum (LGM) the 0 �C isotherm was abovesea-level and so sea ice, although present, wasprobably not extensive (CLIMAP, 1976).

The presence of encrusting biota (serpulids,bryozoans) on clasts and on shells within theconglomerates, in all orientations (including onnested shells; Fig. 8D and F), points to periods ofnon-deposition in the source area, as confirmedby the C. californica microfossil biofacies.

Finally, the biota in fossiliferous conglomerates(dominated by brachiopods) is somewhat differentfrom that in the planar coquinas. It may be that thepalaeoenvironment now represented on the islandwas not one of simple north-inboard and south-outboard polarities, but instead that the island waslocated at a low angle to the shelf edge. In such asituation the northern area was located in shallowwater, whereas the southern area was locateddownslope along the margin of one of the marginalvalleys (cf. Eyles & Lagoe, 1989).

Reworking and redeposition

The coquinas, as partially or completely redepos-ited phenomena, could finally have accumulatedas storm deposits, as seismites, or as wavereworked lags.



Storm depositsThe climate in this region during early Pleisto-cene interglacials generally was similar to thattoday (Eyles & Lagoe, 1989; Lagoe et al., 1989),but somewhat more extreme during glacial peri-ods. In this area, with such a frequency of intensestorms, tempestite deposits would have beenexpected.

Storm depositional processes prior to butunrelated to carbonate deposition are confirmedby the presence of hummocky cross-stratification(HCS) and swaley cross-stratification (SCS) in finesandstones just below Q1 (Fig. 6). In addition, thepresence of interference wave ripples at the samestratigraphic level implies relatively shallowwater.

Storm deposition of the carbonates would haveinvolved a mix of geostrophic currents, waveoscillations and excessive weight forces(cf. Myrow & Southard, 1997). Such deposits arewell-described from the geological record with aset of recurring features (Myrow, 2003). Thepresence of delicate shells such as mussels, andlack of obvious abrasion or breakage of pectens,however, implies short-distance transport. Manyconspicuous attributes of typical storm deposits(e.g. HCS) are, however, not present in theseYakataga coquina deposits because of the extra-ordinarily large particle size.

Normal-graded, gravel-to-sand upward bedsindicate that at least some of the deposits wereemplaced by sediment gravity flows. Whereassuch turbidites occur in many depositional envi-ronments, they are also part of the suite ofsedimentary deposits associated with storms(Myrow & Southard, 1997).

Sediment reworking by storm waves is afundamental attribute of tempestites. The nestedattribute of all bivalves (Fig. 7B and D) andmany brachiopods in these beds testifies tostrong wave sorting and repacking by oscillatorywater motion (Seilacher & Meischner, 1965;Brett, 2003). There is a recurring theme of mostlyconvex-down shells at the base of any unit,variable shell orientation in the middle andconvex-up shells at the top. How this themedevelops is uncertain but may be related to rapiddeposition followed by reworking as sedimenta-tion slows.

Finally, large subaqueous pebble and gravel,two-dimensional decimetre-scale ripples at thetop of units (Fig. 8A) points to strong tractioncurrents during the final stages of deposition.This effect is also suggested by the en echelon,

offlapping arrangement of sandy gravel layers atthe top of some units. In many ways, they are likecoarse-grained hurricane (cyclone)-related depos-its on the Great Barrier Reef (Gagan et al., 1988)wherein storm waves resulted, not in offshore-directed transport, but largely in situ resuspen-sion and settling.

Redeposition related to seismicitySeismites or tsunamites result from transport andreworking by enormous shallow-water waves thatdevelop in the wake of earthquakes or theirrelated landslides, terrestrial and marine. Thisinterpretation is the most difficult to assessbecause there is so little information in theliterature about such deposits in the subtidal,neritic environment (Reinhardt et al., 2006) andthe available information focuses on shorelinesedimentation (Felton et al., 2000; McMurtryet al., 2004). Tsunami deposits should becommon in the geological record (Coleman,1968) but few have been documented withconfidence (Takashimizu & Masuda, 2000; Pratt,2002; Brookfield et al., 2006).

Tsunami waves from offshore or onshore woulddramatically affect the shallow sea floor and,although they might result in total homogeniza-tion of the sea floor nearsurface sediment,subsequent solitons would doubtless rework, sortand impart a wave and current motif to thedeposits. In short, the unit would resemble atempestite except that there would be a relativelyminor geostrophic component to the redeposi-tional process.

On the other hand, repeated episodes of depo-sition at one horizon imply less random, seismi-cally related and more linked processes.Furthermore, almost all reports of seismicallyrelated deposits describe associated sedimentliquifaction features and water escape structures(Seilacher, 1984), none of which are present inthe Yakataga Formation on Middleton Island. Yetit is hard to imagine that there was no effect fromtsunamis in such an earthquake-prone region oversuch a long period of time. Comparison betweenthe coquinas and interpreted Miocene tsunamibackwash deposits in shallow marine settingsfrom Chile (Cantalamessa & Di Celma, 2005) isparticularly striking. Finally, Weiss & Bahlburg(2006) conclude that tsunami waves associatedwith the recent 2005 Sumatra earthquake had thepotential to transport sediment particles fromdecimetres to metres in diameter, similar to thosein the Yakataga Formation.



Condensed horizonsReworked shell horizons can be associated withintense wave activity on extensive shell bedsduring terrigenous clastic sediment starvation(Orphin et al., 1998). Such beds, but without theubiquitous outsized clasts, strongly resemble shellbeds that develop in terrigenous clastic deposi-tional systems during transgression. During Yakat-aga Formation deposition the sea floor would havebeen a site of prolific pecten growth, but clearlysubject to periodic dramatic disturbance.

Modern analogues

Glacial-marine sediments on the modern Arcticand Antarctic sea floor are the combined productof Holocene and late Pleistocene depositionalprocesses (Eyles et al., 1985; Powell & Molnia,1989; Anderson, 1999). Whereas narrow shelvesare covered with muddy sediment, wider shelvestypically are bipartite, with a relatively deep,inner, mud-covered shelf and a comparativelyshallow (< 100 mwd), outer shelf veneered withcoarse siliciclastic sands and gravels with con-spicuous dropstones and biogenic carbonate. Thecoarse carbonate sediments on these outer banksgenerally are relict.

AntarcticaAntarctica superficially has few similarities withthe Gulf of Alaska because it is a truly polarrealm, yet there are important similarities thatbear on the interpretation of Yakataga Formationcarbonates. The shelves are generally deep shelf(up to 600 m) (Anderson & Molnia, 1989) becauseof isostatic depression.

In the Ross Sea, North Victoria Land and WilkesLand, the inner shelf is mostly terrigenous anddiatom mud. The outer shelf banks, shelf edge andupper slope, however, are quite different andcomposed of residual or relict sandy gravel withabundant biogenic carbonate whose componentsare dominated by bryozoans. Most carbonates onthe sea floor (as confirmed by 14C analysis) weredeposited during the LGM (Anderson & Molnia,1989; Taviani et al., 1993; Anderson, 1999) and soare not in equilibrium with modern oceano-graphic conditions. Recolonization of the innershelf by this epifaunal biota did not take placeduring sea-level rise because of the high terrestrialmud influx. Interglacial sedimentation increasedby an order of magnitude due to intense calvingprocesses that delivered a large amount of clayand fine silt (Grobe & Mackensen, 1992).

ArcticIn Arctic outer shelf and shallow seamount envi-ronments where the sea floor lies in the photiczone, epiphytes on kelp and associated barnaclesproduce abundant carbonate sediment (Henrichet al., 1997). Otherwise bivalves, bryozoans andbenthic foraminifera dominate the biota in photicand subphotic environments (Bjorlykke et al.,1978). These latter deposits generally are relictwith a thin veneer of modern carbonate whereinterrigenous mud is winnowed away and bedformsare generated by modern hydrodynamics. Theinner shelf, or a relatively narrow shelf is, how-ever, covered with muddy sediment containing asparse infaunal-dominated, calcareous biota.These relatively modern muddy deposits andtheir biota are thought to reflect the inability ofthe outer shelf epifaunal-dominated biota tomigrate inboard during sea-level rise and glacialretreat because of the high suspended load duringclimatic amelioration.

Gulf of AlaskaThese patterns of surficial sediment likewise arepresent in the Gulf of Alaska. Modern mollusccommunities to the east on the narrow, muddyshelf off Yakatak Bay and Icy Bay (Fig. 1) aredominated by shallow infaunal protobranch,deposit-feeding bivalves (Hickman & Nesbitt,1980). This observation is commensurate withthe muddy, fine-grained character of modernsediment with a Yoldia–Siliqua–Lyonsia commu-nity in silty sand from 22 to 90 mwd, the sameenvironment as the benthic foraminiferalE. excavatum clavatum assemblage (Lagoe et al.,1989). Muds both in fjords and offshore between20 and 172 mwd are characterized by a morediverse Cyclocaria-dominated community. Theseouter, generally subphotic, shelf muds contain aC. norcrossi foraminiferal assemblage.

A much better, but poorly understood, sea flooranalogue of coarse, epifanal mollusc-dominatedsediments seems to be present today atop Mid-dleton Bank, probably because continuous wavewinnowing keeps fine siliciclastic muds fromaccumulating (Sharma, 1979; Powell & Molnia,1989). A similar situation is present on the outerparts of Kodiak Bank some 300 km to the west ofMiddleton Island Bank (Cuffey & Turner, 1987)wherein the shallow bank top is flat-topped, lessthan 100 m deep, is covered with sand andboulders, supports a calcareous, bryozoan-domi-nated biota and is swept clear of fine-grainedsediment by waves and currents.



Synthesis

On balance, Yakataga Formation carbonate depos-its on Middleton Island are interpreted as a seriesof two-phase accumulations. The sediment firstaccumulated in an environment that was condu-cive to biogenic carbonate production but, at thesame time, in a site of ice-borne sediment rainoutfrom mud to boulder-size dropstones. Similaritieswith carbonate deposition on outer shelf offshorebanks during the LGM, both in the Arctic andAntarctic, lend strong support to the notion thatthese sediments were formed near the ice frontduring lowstands and subsequently somewhatstranded during sea-level rise. This environmentwas periodically perturbed by large waves andassociated currents that eroded and redistributedthis sediment. The carbonate sediment factorywas active throughout the period of episodicredeposition. The mechanism of erosion–redepo-sition is uncertain, but was most likely strongstorms or possibly seismic-induced combinedflow. The deposits are, therefore, parautochtho-nous, redeposited sediment from a source area ofactive carbonate production in water somewhatshallower than the deposit itself but not so distantthat carbonate skeletons were fragmented duringtransport.

SEDIMENTATION AND GLACIATION

Introduction

To fully understand the nature of the calcareousdeposits, they must be interpreted in the contextof enclosing strata. Glaciomarine Yakataga For-mation sediments are the product of tectonic,climatic and oceanographic controls. The relativeposition of sea-level was a function of tectonics,glacial loading and rebound, in addition toeustasy. The deposits themselves, however, pro-vide constraints as to the overall setting of theiraccumulation. Current thinking (Eyles & Lagoe,1989) posits that the burrowed mudstones withlocal dropstone layers accumulated on the shelfunder relative sea-level highstands (similar totoday) and all other facies (diamictites, boulderpavements and coquinas) accumulated duringrelative lowstands of sea-level.

The section below Middleton Island is one thatrecords extensive yet localized thrusting, upliftand periodic erosion throughout the Pliocene andPleistocene (Zellers, 1995; Lagoe & Zellers, 1996).Thus, the Middleton Island succession can be

envisaged as having accumulated on an offshorebank possibly surrounded by deeper areas such aslarge channels (cf. Eyles & Lagoe, 1989).

Burrowed and laminated muds (Facies 2)

These sediments, a relatively minor part of thesuccession with carbonates are, as pointed out byEyles & Lagoe (1989), similar to Holocene high-stand shelf muds in the Gulf of Alaska today; theyare relatively thin and comparable with theoutboard parts of the modern seaward-thinningmud wedge. Sediment is intensively bioturbated(Ichnofossil Assemblage IX – Eyles et al., 1992),and contains foraminifera that today live in outershelf to upper slope environments (ca 80 to150 mwd). The contained Cruziana ichnofacies(Table 2) records slow but steady sedimentationand an abundant, diverse infauna; yet they con-tain layers of dropstones, particularly in theupper part of the section (Fig. 4B). Glacier ice istoday grounded onshore, and iceberg transport isminor because not a single glacier extends beyondthe littoral zone, and there is little mud accumu-lating on the outer shelf. However, the system isdynamic because only 55 years ago large numbersof sediment-laden icebergs entered the Gulf ofAlaska from Icy Bay, and dropped large amountsof sand and sandy gravel into the Holocene muds(Anderson & Molnia, 1989). Moreover, a centuryago one glacier extended onto the shelf andcalved icebergs into the Gulf. Thus, these drop-stone layers in the Yakataga Formation muds areinterpreted as representing similar such icebergepisodes.

Diamictites (Facies 3)

Diamictites indicate massive input and rapiddeposition of poorly sorted muddy sediment.Benthic foraminifera (E. excavatum f. clavatumbiofacies) indicate shelf (10 to 80 mwd) environ-ments. Sediments generally are devoid of ichno-fossils except at the top directly beneath boulderpavements or carbonates. Lack of any pronouncedbedding entails continuous deposition from rain-out and the absence of waves or currents ofsufficient strength to sort the sediment. In thisstormy environment, such attributes imply thateither sea ice or floating ice dampened hydro-dynamic energy or that the sea floor was belowstorm wave base. Dropstones indicate melting ofdirty icebergs. What is less clear is the deposi-tional role of these episodic ice covers. The well-preserved microfauna and occasional macrofauna



in diamictites on Middleton Island indicate thatthe deposits originated largely by the rainout ofice rafted debris and mud supplied by meltwaterplumes. Strictly speaking such deposits are nottillites, having been deposited subaqueouslythrough a water column beyond the immediateice margin. Occasional slump horizons and localdeformation of sediments immediately aboveboulder pavements (at the base of overlying deepwater muds) and within diamictites may indicateeither restricted subglacial deformation, bull-dozing and slumping at the ice front or groundingline, or turbation by the keels of ice bergs. It hasbeen speculated that ice did not remain long onthe shelf given the sensitivity of ice to waterdepths; ice lobes probably lifted off and broke uprapidly at the end of each glacial advance cycle asglacioisostatic depression deepened the shelf(Eyles & Lagoe, 1990).

The simplest explanation is that glacial ice wason the shelf, but not at the shelf edge. Calving ofdirty icebergs and bergy bits was widespreadwith the terrigenous clastic source area close toMiddleton Bank. In short, the depositional envi-ronment is considered to have been like theinner part of the shelf today except that fallout offine to very coarse material from icebergs wasprolific.

Boulder pavements (Facies 4)

Striated boulder pavements, consisting of unitsone clast in thickness with faceted upper surfacesindicating unidirectional north to south icemovement, document winnowing of diamict bywaves and tidal currents in relatively shallowwater similar to lags found on shallow-watershoals and banks on the modern Gulf of Alaskashelf (e.g. Tarr bank; Molnia & Carlson, 1980) thatwere subsequently overridden by a grounded iceshelf (Eyles, 1988, 1994). Such pavements arecritical because they are conclusive evidence forgrounded ice (presumably piedmont ice lobes)reaching the outer shelf on successive occasions.

These pavements typically define planar-bounding surfaces at the top of a diamictite. Thetrace fossil assemblage in diamictite directlybelow boulder pavements (Assemblage VII; Eyleset al., 1992) is characterized by sharp-walledburrows (Table 1), confirming a firm but unlith-ified substrate, commensurate with winnowingand removal of fine material to form the boulderlag. Foraminiferal biofacies indicate a shallow-water, inner neritic environment (Lagoe et al.,1989).

Planar coquinas (Facies 5)

Planar coquinas indicate a source area that wassomewhat starved of fine terrigenous clastic sedi-ment and had high epibenthic productivity. Thebiota confirms comparatively shallow shelfdepths (< 80 mwd) and active water movement.Lack of abundant sand and silt implies that either:(i) the sediment bypassed via deeper channels oneither side of the offshore bank (cf. Eyles & Lagoe,1989); (ii) the sediment source was cut off bymoving it landward; or (iii) the fines were win-nowed away. Like boulder pavements, coquinasare interpreted as having originally formed inrelatively shallow water (similar to offshore bankstoday), but near an ice front that supplied verycoarse to fine siliciclastic sediment. Most of thefine terrigenous clastic sediment is thought tohave been swept away and the water column keptrelatively clear by vigorous waves and tidal cur-rents with only the cobble and boulder-sizecomponents, together with trapped interstitialmud, left as a lag. In this interpretation, sea-levelwas relatively low and the ice front was nearby.

Lack of phototrophs in these relatively shallow-water deposits is, however, troubling. The mostlikely explanations are that: (i) the environmentwas dark and lay beneath an ice shelf; (ii) therewas so much suspended mud in the water columnthat sea floor illumination was reduced; (iii) therewas not much mud because the marine ice sheetwas so far from land and there was not muchmelting; or (iv) the environment was deep enoughto be subphotic. An ice shelf is unlikely because ofthe relatively warm waters of the Alaska Currentat or close to the shelf edge. The problem ofsuspended muds is difficult but, given the overallcold conditions associated with maximum glaci-ation, they should be present but not abundant.Submergence to subphotic but still neritic depthsis also appealing because it does not call forspecial conditions except sea-level rise. On bal-ance, the carbonate factory is interpreted ashaving been in an energetic hydrodynamic envi-ronment wherein suspended sediment reducedlight penetration somewhat and that, as sea-levelrose during warming and ice sheet retreat, the seafloor fell below the photic zone. The deposits are,therefore, both lowstand and transgressive.

Channellized fossiliferous conglomerates(Facies 6)

There is not as much detailed microfossil orichnofossil information on these generally finer



grained, more siliciclastic, and less calcareousdeposits, but many of the same constraints apply.The fossiliferous conglomerates are overall dee-per water than coquinas and interpreted as hav-ing formed in upper slope environments withsome of the terrigenous clastics coming fromwinnowed shallow-water environments on banktops. The more terrigenous clastic content of thechannellized fossiliferous conglomerates sup-ports the notion that some of the siliciclasticmaterial was shed into deeper channels that cutthrough the shelf (Fig. 1).

Interpretation

Although general stratigraphy is clear (Plafkeret al., 1994), it is difficult to correlate subsurfaceunits in this region, and thus Zellers (1995) hasconcluded that major fluctuations in bathymetry(< 200 m) were mainly the result of basementuplift/subsidence and sediment loading, but noteustasy. While this is undoubtedly true, recurringstratigraphic patterns that can be explained bylinked climate and sea-level excursions implythat at least a first-order explanation can be

related to variations in a glacioeustatic sea-levelcycle (Fig. 11).

In such an interpretation (Fig. 12), the inter-glacial highstand massive burrowed muds(Fig. 12A) are interpreted easily as similar toHolocene highstand deposits in the Gulf ofAlaska (Eyles & Lagoe, 1989) which means that,at interpreted palaeowater depths of 80 to 150 m,Middleton Island did not exist as a bank like itdoes today with water depths of < 50 m over wideareas, but was more like the shelf to the south-eastoff Icy Bay and Yakutat Bay where the shelf edgeis close to 200 mwd.

Marine diamicts (Fig. 12B) record massiveinput of muddy sediment, as well as prolificrainout of sand and boulder dropstones fromicebergs. At the same time deposition clearly tookplace below storm wave base, implying relativelydeep shelf environments. Whereas hydrodynam-ics could have been attenuated somewhat byfloating ice, the climatic conditions of the Gulfargue against a floating ice shelf. Omissionsurfaces at the top of diamict units, overallcoarsening-upward, and presence of storm-gener-ated sedimentary structures in the upper parts

Fig. 11. Interpretation of the relationship between sedimentary facies in the middle and upper Yakataga Formationon Middleton Island and climate-related glacio-eustatic fluctuations in sea-level. Shaded region of curves representstime in which carbonates are thought to have been deposited.



A

C

B

D

Fig. 12. Sketch illustrating the spatial relationships between glacial position, sea-level and sediment production atthe positions illustrated in Fig. 11.



of some units support this interpretation andindicate overall shallowing into storm wave base.

The stratigraphic attributes of boulder pave-ments and coquinas are similar in that both occurabove diamicts where the ichnofacies (Table 1)indicate omission, and both are overlain bydiamict. Furthermore, in both cases, the coarsenature of the deposits strongly suggests winnow-ing. The simplest explanation is that in the case ofboulder pavements winnowing was followed byice overriding the boulders (Fig. 12C), whereas inthe case of coquinas winnowing was coeval withor followed by biotic growth during initial sea-level rise and ice meltback (Fig. 12D). The impli-cation is that carbonates formed in relativelyshallow water near the ice front, with waves andcurrents sweeping away most of the mud andkeeping the waters clear. The sediment factory(Fig. 13) would have been near the ice front andconsisted of a bouldery sea floor of dropstoneswith attendant fine material winnowed away bywaves and currents, much as it is today onMiddleton Shelf, Tarr Bank and Kodiak IslandBank. The epifaunal biota was subphotic withphosphate suggesting ice front upwelling and

good trophic resources. The relative shallownessof the sea floor made it susceptible to exception-ally violent storms and tsunamis, for whichthe Gulf of Alaska is famous, and episodicredeposition.

Thus, diamicts would have formed when waterover the shelf was relatively deep and glacial icewas on the shelf but not at the edge (Fig. 13),pavements and carbonates formed when ice wasnear the shelf edge and the water was somewhatshallower, in the zone of waves and tides. Boththe pavements and carbonates were strandedduring subsequent sea-level rise.

In such a hypothesis, muds represent inter-glacials, diamicts represent periods when the icewas on the shelf and carbonates and boulderpavements represent glacial periods and initialmelting. As such, boulder pavements recordintense glacial periods whereas carbonates indi-cate less severe glaciations. Placed against thestratigraphic column (Fig. 3), this hypothesissuggests an early period with more extensiveglaciations (lower part of the section) followed bya later period of less intense glaciations (upperpart of the section).

Fig. 13. Sketch illustrating interpreted position of the carbonate factory and resulting redeposited carbonates placedagainst the location of Middleton Island.



In the final analysis, these carbonates illustratethe axiom that calcareous sediment will accumulatein the neritic realm whenever siliciclastic sedi-mentation is reduced (James & Kendall, 1992),regardless of latitude. In this case, siliciclasticaccumulation was inhibited by cold conditionsand energetic hydrodynamics associated withglacial maximums and ice advance across theAlaskan shelf. Thus, carbonates formed within awindow of opportunity. The situation prevailedduring subsequent sea-level rise associated withwarming until muds or diamictites associated withhighstand or readvance buried the carbonates.

DISCUSSION

Carbonates in the modern polar realm

James (1997), stressing that the database was small,subdivided high-latitude heterozoan carbonatesinto sub-polar and polar associations. Sub-polar orcold-temperate sediments accumulate in oceansthat are only seasonally ice covered, summer watertemperatures are 2 to 6 �C and the environment issubject to seasonal storms and strong currents. Bycontrast, polar carbonates form in contact with ice,in waters that are < 5 �C, are associated withbiosiliceous deposits, locally contain glendonitesand are typified by gigantism.

Using this scheme, Yakataga carbonates are sub-polar, in keeping with their relatively low latitudeof ca 60� N. The attribute that stands out withYakataga Formation sediments, however, is thecomparatively large amount of mud depositedduring almost all parts of the glacio-eustatic sea-level cycle. This deposition is because the envi-ronment was adjacent to fast-flowing wet-basedglaciers eroding a high, mountainous relief alongthe Gulf of Alaska coastline. There is also noevidence of a perennial ice shelf, although glaciersdid periodically extend to the shelf edge. Theterrigenous sediment was transported onto most ofthe shelf by suspension during interglacials and bymelting of glacial ice and icebergs on the shelfitself. Higher latitude shelves with extensive sea-sonal ice but not glacial ice today are simplyveneered with mud with few dropstones (Sharma,1979), quite unlike the Yakataga Formation.

Phanerozoic sub-polar carbonates

CenozoicCenozoic, post-Eocene, icehouse, glacial-marinesediments and sedimentary rocks are not

well-represented in the stratigraphic recordbecause most are still buried beneath continentalshelves. One such Oligocene deposit that containscarbonates is, however, exposed on King GeorgeIsland, South Shetland Islands, West Antarctica(Gazdzicki, 1984; Porebski & Gradzinski, 1987).The stratified conglomerates, sandstones andshell beds are astonishingly similar to parts ofthe Yakataga Formation described herein. Similarpecten coquinas are composed of innumerableChlamys anderssoni and less numerous Mytilus,interpreted as tempestites. Beds are sharplyerosional based, 5 to 25 cm thick, with shellsoriented parallel to the base, commonly convex-upward and locally graded. Biota includescoccoliths, planktonic and benthic foraminifera(Nodosariidae, Miliolidae and Elphididiidae), cal-careous worm tubes (spirorbids and serpulids),bryozoans (49 species, mainly encrusting formson clasts, shells and worm tubes) bivalves (28species), gastropods (with prominant Natica, apredator) ostracods, as well as crinoids andechinoids. Deposits are interpreted as a mixtureof fauna from two different environments; they arewithin an otherwise glaciomarine succession withconspicuous dropstones with units containingboulder pavements.

Shells are densely packed and mostly horizon-tal, both concave-up and concave-down andlocally vertical. Some beds show an upwardchange from horizontal to vertical shell orienta-tion. Whereas two identical examples separatedby several million years from the same generalpalaeoenvironment do not make a trend, theirsimilarity suggests that there may be a recurringfacies style for such settings.

PalaeozoicThe two major Palaeozoic glacial periods are theLate Ordovician and the Permo-Carboniferous.Whereas Late Ordovician glaciomarine sedimentsare poorly known, those from the Late Palaeozoicare much better documented and many containcarbonates.

Amongst the better understood of these south-ern hemisphere carbonates are those in western(Eyles et al., 2006) and south-eastern Australia(Draper, 1988; Dickens, 1996; Rogala et al., 2007)that accumulated during prolonged early to mid-dle Permian deglaciation. Those carbonates, withlocally numerous glacial dropstones, are com-posed of a relatively low diversity but abundantbiota dominated by bryozoans, molluscs, brachio-pods and crinoids. Although clearly Palaeozoic incharacter, this association is particularly rich in



numerous bivalves, especially pectenids (e.g.Deltapecten) and pectenoids (e.g. Eurydesma)(Newell & Boyd, 1995). A recurring theme is thatthe carbonates accumulated whenever glacial-marine siliciclastic sedimentation was, for what-ever reason, attenuated.

When differences due to evolution are takeninto account, particularly the Mesozoic increasein predation and grazing pressure resulting ininfaunalization (Vermeij, 1977, 1987), thesecarbonate-rich sediments are strikingly similarto Cenozoic and modern high-latitude glacioma-rine deposits. The implication is that the mollusc-dominated, epifaunal, sub-polar biota thatincludes the characteristic heterozoan assemblagewas in place during the post-Carboniferous south-ern hemisphere glaciation meltdown. It is not thatthe modern biota is in part archaic (retrograde), assuggested by Gili et al. (2006), but that the polarand sub-polar neritic epifaunal biomineralizedassemblage evolved during this last pre-Cenozoicglaciation.

A caveat is that Yakataga Formation sedimentscontain no glendonites indicative of subfreezingsea water (Kaplan, 1977; Shearman & Smith,1985; DeLurio & Frakes, 1999; Swainson & Ham-mond, 2001); the Permian glacial-marine sedi-ments (but not the carbonates) from Australia docontain glendonites. This effect probably reflectsthe minimal upwelling, dominant downwelling,somewhat warmer temperatures and relativelylow nutrient levels compared with environmentsof ikaite formation.

SUMMARY AND CONCLUSIONS

1 Uppermost Plio-Pleistocene Yakataga Forma-tion glaciomarine sedimentary rocks are, as aresult of uplift during the 1964 earthquake, ex-posed on the extensive intertidal rocky flats ofMiddleton Island in the Gulf of Alaska. Thesewell-studied deposits provide a unique windowinto an otherwise inaccessible succession ofcontinental margin glacial and interglacial dia-mictites, mudstones and carbonates thataccumulated in a high-latitude depositionalrealm.

2 Metre-scale-thick carbonates occur as:(i) planar coquinas; and (ii) fossiliferous chann-ellized conglomerates. Planar coquinas comprisestacked, channellized, boulder conglomeratesrich in bivalves (especially pectens), gastropods,benthic foraminifera, bryozoans and serpulids.Many beds are normally graded, whereas most

shells are nested. Shells and lithic clasts withinunits commonly are encrusted with bryozoansand serpulids. Fossiliferous channellized con-glomerates are less fossiliferous, less calcareous,thinner, not as laterally persistent and, althoughthey contain the same components, are conspic-uously richer in brachiopods.

3 The carbonates are two-phase deposits. Thesediments originally accumulated in an ice-proximal setting that was periodically perturbedby storms such that the sediments were episodi-cally eroded and redeposited.

4 The original carbonate factory was locatedin a relatively shallow-water (ca 80 m waterdepth) environment. The biota confirms a rela-tively clear water, cold (< 5 �C), mesotrophic,subphotic setting. The calcareous invertebratesthrived on a boulder-rich sea floor that was keptfree of mud by constant wave and tidal-currentactivity.

5 Many attributes suggest that the deposi-tional mechanisms that led to the coquinas aspresently constituted were storms and, giventhe size of outsized clasts, they must have beenexceptionally intense. It is also possible, how-ever, that some of the redepositional eventswere the result of wave motion generated byseismic events, i.e. tsunamis, in this area ofongoing tectonic activity.

6 Placed in the context of Plio-Pleistoceneglacio-eustasy, the different facies can be inter-preted as products of complex climate, sea-level, glacial and biotic controls. Interglacialconditions were much like those today whereinglaciers were trapped inboard, limited to thelittoral zone, discharging enormous amounts ofterrestrial silt and clay (glacial flour) onto theshelf. This sediment was, in turn, redistributedby ocean currents and resulted in a seaward-thinning wedge of structureless and burrowedmuds. Diamictites are interpreted as havingaccumulated during glacial advance. By con-trast, glacial episodes are recorded by boulderpavements formed by winnowing and followedby grounding of glacial ice in shallow water atthe edge of the shelf. Carbonates formed undersimilar conditions but during those times whenglacial ice did not extend to the shelf edge, muddeposition was arrested by constant winnowing,and there was minor but significant ice-frontupwelling.

7 When compared with other high-latitudemodern and Phanerozoic glacially influencedcarbonates, there are tenuous but compellinglinks that extend to the late Palaeozoic. Most such



deposits are rich in epifaunal molluscs and par-ticularly pectenoids, with recurring bryozoansand benthic foraminifera. Thus, it is proposedthat the modern warm-polar, glacial marine car-bonate biota is a product of late Palaeozoic evo-lution related to the Permo-Carboniferoussouthern hemisphere glaciation.

8 The rocks described herein are unique in thatthey are not simply glacial influenced, in thesense that there were icebergs and sea-ice presentin the vicinity during deposition, but the car-bonates were initially deposited adjacent to amarine ice sheet and stranded during subsequentretreat and sea-level rise. The carbonates formedduring glacial periods.

ACKNOWLEDGEMENTS

This research is funded by the Natural Sciencesand Engineering Research Council of Canadagrants to NPJ, CHE, NE and TKK, and by aUniversity of Wisconsin-Oshkosh Faculty Devel-opment Research Grant to EEH. We thank Y. Boneand P. Taylor who identified bryozoans andhelped with their interpretation, R.M. Carter forhis advice on molluscs in general, M. Claphamwho sharpened our understanding of Palaeozoicbivalves, B. Jones who aided in the interpretationof endoliths, and G. Jaecks for help with thebrachiopods. I. Malcolm assisted with all techni-cal aspects of the study. The manuscript wasgreatly improved by insightful comments byJ. Anderson, R.W. Dalrymple, G.R. Dix and T.R.Frank.

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