Depositional processes on oceanic island shelves–Evidence from storm‐generated Neogene deposits...

Depositional processes on oceanic island shelves – Evidencefrom storm-generated Neogene deposits from the mid-NorthAtlantic

RICARDO P. MEIRELES*†‡ , RUI QUARTAU§ , RICARDO S. RAMALHO¶**,ANA C. REBELO*†‡ , JOSE MADEIRA†† , VITTORIO ZANON‡‡ andSERGIO P. AVILA*†‡§§*CIBIO, Centro de Investigacao em Biodiversidade e Recursos Geneticos, InBIO Laboratorio Associado,Polo dos Acores, Acores, Portugal (E-mail: [email protected])†Departamento de Biologia, Universidade dos Acores, 9501-801 Ponta Delgada, Acores, Portugal‡MPB-Marine PalaeoBiogeography Working Group of the University of the Azores, Rua da Mae deDeus, 9501-801 Ponta Delgada, Acores, Portugal§Divisao de Geologia e Georecursos Marinhos, Instituto Portugues do Mar e da Atmosfera, Rua C doAeroporto, 1749-077 Lisboa, Portugal¶Institut fur Geophysik, WWU Munster, Corrensstraße 24, 48149 Munster, Germany**School of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS81RJ UK††Departamento de Geologia, Faculdade de Ciencias da Universidade de Lisboa, Instituto Dom Luiz(LA), 1749-016 Lisboa, Portugal‡‡Centro de Vulcanologia e Avaliacao de Riscos Geologicos, Universidade dos Acores, 9501-801 PontaDelgada, Acores, Portugal§§Faculdade de Ciencias da Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Associate Editor – Jess Trofimovs

ABSTRACT

Oceanic islands – such as the Azores in the mid-North Atlantic – are period-

ically exposed to large storms that often remobilize and transport marine

sediments along coastlines, and into deeper environments. Such disruptive

events create deposits – denominated tempestites – whose characteristics

reflect the highly dynamic environment in which they were formed. Tempes-

tites from oceanic islands, however, are seldom described in the literature

and little is known about storm-related sediment dynamics affecting oceanic

island shelves. Therefore, the geological record of tempestite deposits at

oceanic islands can provide invaluable information on the processes of sedi-

ment remobilization, transport and deposition taking place on insular

shelves during and after major storms. In Santa Maria Island (Azores), a

sequence of Neogene tempestite deposits was incorporated in the island

edifice by the ongoing volcanic activity (thus preserved) and later exposed

through uplift and erosion. Because it was overlain by a contemporary

coastal lava delta, the water depth at the time of deposition could be

inferred, constituting an excellent case-study to gain insight on the still enig-

matic processes of insular shelf deposition. Sedimentological, palaeontologi-

cal, petrographic and palaeo-water depth information allowed the

reconstruction of the depositional environment of these sediments. The

sequence typifies the characteristics of a tempestite (or successive tempes-

tites) formed at ca 50 m depth, in a steep, energetic open insular shelf, and

with evidence for massive sediment remobilization from the nearshore to the

middle or outer shelf. The authors claim that cross-shelf transport induced

© 2013 The Authors. Journal compilation © 2013 International Association of Sedimentologists 1

Sedimentology (2013) doi: 10.1111/sed.12055

by storm events is the main process of sediment deposition acting on steep

and narrow shelves subjected to high-energetic environments, such as the

insular shelves of open-sea volcanic islands.

Keywords Depositional model, insular shelf, palaeo-water depth, sedimenttransport, storm-induced flows, tempestite.

INTRODUCTION

Oceanic islands are, by nature, exposed to opensea conditions and thus affected by severecoastal erosion. Their coasts are continuallybeing cut back by the sea, producing largeamounts of sediments, but clastic depositionalcoasts are rarely seen on these islands. There isalso a clear lack of knowledge concerning sedi-mentation processes on rocky shoreline envi-ronments, such as those of volcanic islands(Felton, 2002). The few published studies arerestricted to nearshore settings in mixedsiliciclastic–carbonate systems (Schneidermannet al., 1976; Morelock et al., 1983; Schwab et al.,1996; Harney et al., 2000; Calhoun et al., 2002;Harney & Fletcher, 2003; Ogston et al., 2004;Storlazzi et al., 2004; Conger et al., 2009; Ryan-Mishkin et al., 2009). Unfortunately, these stu-dies focused mainly on sediment characterizationrather than on processes of deposition. Based ontheir observations, Tsutsui et al. (1987), Chiocci& Romagnoli (2004) and Quartau et al. (2012)suggested that bottom downwelling currents,generated during storms, transport nearshoresediments perpendicular to the coast onto theshelf and slope environments, building sandydeposits offshore. However, this model is contro-versial because, although the geological record ofstorm deposits confirms this hypothesis, modernoceanographers find it difficult to explain howpurely unidirectional flows can cross the entireshelf. Therefore, models of tempestite depositionare still under debate, with scientists describingdensity-induced flows, wave oscillation flows,geostrophic currents and combinations betweenthese three (Myrow & Southard, 1996; Myrow,2005) as possible transport processes. Thus, thekey to understanding the mechanisms behindisland shelf sedimentation is to look back intothe geological record, as was the case for thisstudy.The Azores Archipelago – located in the mid-

North Atlantic – is often subject to violentstorms (Borges, 2003; Andrade et al., 2008) asso-

ciated with remarkable wave energy (Quartauet al., 2012; Rusu & Guedes Soares, 2012). Thisarchipelago is a group of very young oceanicislands, featuring youthful volcanic landformsthat are generally devoid of exposed marinevolcanic and sedimentary sequences. The east-ernmost island of Santa Maria, however, is anexception to this scenario. The island is mucholder than the remaining – having emergedduring the Late Miocene (Abdel-Monem et al.,1968, 1975; Feraud et al., 1980, 1981) – and isrich in Neogene submarine volcanic sequencesand fossiliferous marine sediments, due to acombination of coastal erosion and uplift duringthe Plio-Quaternary (Serralheiro et al., 1987;Serralheiro & Madeira, 1990; Serralheiro, 2003;�Avila et al., 2012). Santa Maria is, thus, a primelocality for the study of tempestite depositsallowing insight into sediment dynamics takingplace on exposed oceanic island shelves. Thisstudy focused on the sedimentary outcrops atPonta do Castelo, in south-east Santa Maria,because the overlying volcanic sequence – a lavadelta – enables accurate interpretation of the pal-aeo-water depth for the top of the sedimentarypackage. A comprehensive characterization ofthe outcrop from volcanostratigraphic, sedimen-tological, palaeontological and petrographicalpoints of view allowed reconstruction of the pal-aeo-environment of deposition. In this study,supporting evidence is provided that storm-induced downwelling currents have transportedsediments from the nearshore to deeper offshoreenvironments. It is further suggested that this isthe main process of deposition on steep, narrowand high-energetic shelves, such as those foundon oceanic islands.

GEOLOGICAL SETTING

The Azores archipelago is a group of nineislands located in the North Atlantic (25° to32° W; 37° to 40° N), in a complex tectonicsetting. Two of the Azorean islands – Flores

© 2013 The Authors. Journal compilation © 2013 International Association of Sedimentologists, Sedimentology

2 R. P. Meireles et al.

and Corvo – rise from the western flank of themid-Atlantic Ridge, whilst the remaining sevenislands – including Santa Maria – are locatedto the east of this feature along the westernsegment of the Eurasia–Nubia boundary(Fig. 1A).

Santa Maria is the easternmost island of theArchipelago and, supposedly, was the first tohave emerged during the Late Miocene, sometime before 8�1 Ma (Abdel-Monem et al., 1975;Serralheiro & Madeira, 1990; Serralheiro, 2003).The general volcanostratigraphic sequence of

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Fig. 1. Location maps: (A) of Santa Maria, within the Azores Archipelago, MAR – Mid-Atlantic Ridge, AM –American Plate, EU- European Plate, AF – African Plate; (B) of Ponta do Castelo on south-east Santa Maria (‘1’Rocha Alta); (C) detailed image of the outcrop area (‘1’ Ponta do Castelo; ‘2’ Pedra-que-pica).


Depositional processes on oceanic insular shelves 3

Santa Maria, as defined by Serralheiro et al.(1987), basically reflects (see Serralheiro & Madeira,1990; Serralheiro, 2003; �Avila et al., 2012): (i)the emergence of a volcanic edifice during theLate Miocene (the Cabrestantes and Porto For-mations); (ii) the construction of a basalticshield volcano during the Late Miocene (theAnjos Complex); (iii) subsequent erosion andprobable total immersion of the shield volcano,sedimentation of marine and terrestrial sedi-ments, with synchronous submarine volcanicactivity on the eastern side of the island, duringthe Late Miocene/Early Pliocene (the TourilComplex); (iv) an increase in volcanic activity –initially exclusively submarine and later subaer-ial with the formation of lava deltas along coevalcoastlines – and re-emergence of the edifice dur-ing the Early Pliocene (the Facho – Pico AltoComplex); and (v) erosion followed by low vol-ume volcanic activity, forming a set of mono-genetic magmatic and hydromagmatic cones,during the Late Pliocene (the Feteiras Forma-tion); uplift and erosion of the edifice from LatePliocene to the present.Ponta do Castelo is a tall prominent headland

forming the south-easternmost tip of the island(see Fig. 1B and C). In this location, thesequence comprises (Fig. 2), from the base to the

top: (i) fossiliferous marine sediments with highvolcaniclastic content; (ii) a typical lava deltasequence constituted by a steeply dipping fore-set unit of basaltic pillow-lavas and hyaloclas-tites, and a topset unit of flat-lying subaerial lavaflows; and (iii) basaltic pillow-lavas (where thelighthouse stands), part of another lava deltasequence stacked on top of the first and whosetopset unit has been removed locally by erosion.The foresets of pillow-lavas and hyaloclastites ofthe main lava delta sequence dip 25° to 35° tothe east, suggesting a coeval coastal progradationin this direction, under moderate extrusionrates, and over a shelf where the sedimentsrested. The continuous exposure along the coasttowards the west shows that the sediments atPonta do Castelo partially correspond to the re-mobilization of tuffs and water-settled tuffitesfrom the surtseyan cone whose remains can stillbe seen further west at Rocha Alta. The overallsequence is attributed to the Facho – Pico AltoComplex for which an Early Pliocene age hasbeen suggested (Serralheiro et al., 1987). Effec-tively, the underlying sediments at Pedra-que-Pica (a nearby sedimentary outcrop 800 m to thewest; see Fig. 1C) yielded an isotopic age around5�5 Ma (Kirby et al., 2007), thus supporting thisage estimate.

Fig. 2. General cross-section of the Ponta do Castelo volcano-sedimentary sequence, showing the volcanostrati-graphic setting of the basal sedimentary sequence (tempestite deposit), and the location of the passage zonebetween subaerial and submarine flows (representing coeval sea-level) within the overlying lava delta sequence;‘d’ represents the contemporaneous palaeo-water depth for the top of the sediments, as inferred from the overlyinglava delta structure; apsl, above present sea-level.



METHODS

Sedimentological and palaeontologicalcharacterization

The outcrop at Ponta do Castelo was studied indetail to reconstruct the overall structure,geometry and field relations between the sedi-mentary deposit and the underlying/overlyingvolcanic sequences. Two cross-sections weremade along the south side of Ponta do Castelo: ageneral section of the sequence (Fig. 2); and adetailed composite section (Fig. 3) showing thearrangement, internal structures and erosionalcontacts of the tempestite deposit. Strip logs forstratigraphic sections (Fig. 4) represent the vari-ability and facies succession found at this outcrop.Special care was taken to record the geometry anddimensions of the observed sedimentary struc-tures, the lateral and vertical continuity of facies,and the position and taphonomical aspects of thefossil content. Macrofossil content was verified insitu, carefully recorded and classified to the genusand species level, whenever possible. Thin sec-tions, representative of the main subunits withinthe deposit were studied under polarizingmicroscopy to infer structures, micro-textures,mineralogy and microfossil content. All fossilspecimens collected in this study are housed inthe Fossil Collection of the Department of Bio-logy of the University of Azores (DBUA-F).

Palaeo-water depth estimations using thevolcanic structure

Coastal lava deltas constitute excellent sea-leveltracers because the passage zone between thetopset unit (composed by flat-lying subaerial lavaflows) and the foreset unit (composed by pro-grading foresets of submarine lavas) very accu-rately marks the position of coeval relative sea-level (Jones & Nelson, 1970; Porebski & Gradzinski,1990; Smellie, 2000; Immenhauser, 2009; Ramalho,2011). Therefore, the vertical distance betweenthe base of a foreset and the passage zone at itstop can be used to estimate very accurately thewater depth at the time of extrusion (see Fig. 2).If pillow-lavas from the foreset unit cast loadmarks on the underlying sediments, it meansthat these sediments were still soft when theywere covered by the effusive sequence. Thus, itcan reasonably be assumed that the palaeo-waterdepth inferred for the base of the volcanicsequence corresponds to the water depthcontemporaneous to the top of the underlying

sediments. Furthermore, if sedimentation rateswere high and sea-level did not change duringdeposition, palaeo-water depth information maybe transposed into the sediments below. In thepresent study, vertical distances were measuredwith an Impulse 200LR laser distance meter pro-duced by Laser Technologies, Inc. (LTI, Centen-nial, CO, USA), with a range up to 500 m.

RESULTS

The studied sequence is outstanding for its char-acteristics, even within the island context. Someof its most striking features are the mega-ripples(hummocks) that can be seen in the basal sedi-ments and the very clear lava delta structure over-lying the sediments (see Figs 2 to 5). Theoutcrops are partially truncated by a Late Pleisto-cene shore platform, beach and former cliff, withan age that probably corresponds to the last inter-glacial [Marine Isotopic Stage (MIS) 5e, ca 115 to135 ka] (�Avila et al., 2008); these were later cov-ered by slope deposits (constituting the talus/col-luvial fan where the former whaling station islocated). The geometry of the sedimentary bodyseems to be sigmoidal or wedge-like, with the topdipping about 15° to the ENE and located at amaximum elevation of ca 9 m above present sea-level (apsl). The base of the sequence is buried bythe modern beach; however, nearby outcrops ofunderlying lavas and the geometry of thesequence suggests that it is not far from the sur-face. The overlying lava delta exhibits a passagezone at ca 55 m apsl, suggesting a palaeo-waterdepth of 45 to 55 m for this deposit (‘d’ in Fig. 2).The sedimentary sequence is entirely com-

posed of fossiliferous sandstones that resultedfrom the remobilization of hydromagmatic tuffs.Hence, grain composition is dominated by angu-lar or poorly rounded, glassy, non-vesicular andpalagonitized volcanic clasts (ca 58%), with areduced amount of mineral clasts (olivine, pyrox-ene, plagioclase, tiny cubic oxides and calcite),fossil shell fragments (amounting to ca 20% andcomprising gastropods, bivalves, rhodoliths, echi-noids, bryozoans, corals and balanids) and micro-fossils (foraminifera and ostracoda), all cementedby calcite (sparite) – see Fig. 6. However, despitethe homogeneous lithology, the deposit showslarge lateral and vertical variations in the sedi-mentary structure.The exposed lower part of the sequence (hence-

forth called ‘Unit 1’) is composed of coarse tomedium-grained sandstones up to 1 m thick,



almost devoid of macrofossils, and showing awide, large-scale cross-stratification with centi-metre to decimetre-thick slightly wavy sets, form-ing low but wide swales and possibly hummocks(see Figs 3, 5A and 5B). There is some weaknormal grading within individual beds. The baseof the deposit is not exposed and the small extentof the outcrop precludes any solid large-scale geo-metric reconstruction. However, it is possible thatthe observed features are actually part of a largerhummocky or swaley cross-stratified structure.Petrographic investigation revealed it to be com-posed mainly of rounded to angular glassy shardswith a few subrounded lava lithics. Mineral clastsand fossils are rare (Fig. 6A).Above the previous sediments, sits a wedge-

shaped sedimentary package (henceforth called‘Unit 2’) that gradually loses thickness towardsthe ENE (i.e. from ca 1�20 m to ca 0�5 m) andexhibits a pronounced vertical variation in

sedimentary structures and in grain size (seeFig. 3). This unit is bounded by erosional un-conformities: the lower unconformity is irregu-lar, forming a centimetre to decimetre-highpalaeo-topography, while the upper unconfor-mity is more regular and parallel to bedding,with the exception of a shallow but wide palae-o-channel (see Fig. 5C). Unit 2 gradually gradesfrom micro-conglomerates/coarse sandstones atthe base, into coarse to medium-grained sand-stones (Fig. 6B) with chaotic/diffuse bedding inthe middle (both very rich in shell debris), theninto medium to fine-grained sandstones withplane-parallel bedding/lamination at the top,composed of alternating bioclastic-rich andvolcaniclastic-rich laminae. Two species ofostracoda were found within bioclastic-richlaminae: Pachycaudites aff P. armilla Ciampo,1986 and Neonesidea rochae Nascimento, 1989.Small-scale bioturbation (Diplocraterion isp.)

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Fig. 3. Sedimentary/tempestite deposit at Ponta do Castelo, SE Santa Maria. (A) Photograph of the main outcrop(western portion of the outcrop), with people for scale (ca 1 8 m tall) and main erosional contacts marked asdashed lines. Location of strip logs are labelled in the picture; Log 3 was performed to the E of this picture,behind the fallen boulders. (B) Simplified cross-section of the studied sediments, representing main lithologies,sedimentary structures, contacts, fossiliferous content and the location of strip logs. Main erosional unconformi-ties are represented by thicker solid black lines and numbering corresponds to the described depositional units.



interrupts the topmost beds/laminae. The mid-dle subunit (with chaotic/diffuse bedding)increases in thickness towards the WSW and theupper subunit (with plane-parallel bedding/lam-ination) is absent towards this direction – proba-bly removed by the subsequent erosion thatcreated the upper unconformity. Thin-sectionobservations show that constituents are some-what different from those present in Unit 1.Glassy shards of volcanic origin are less abun-dant than the lava lithics, but very large (up to0�5 cm across) palagonite clasts are present.Lithics are represented by small rounded vesicu-lar and porphyritic pyroclasts. Mineral clastsand fossils are also more abundant. It is evidentthe presence of a first thin sparite rim around allclasts, and a second sparite cement filling all thepores, suggesting the occurrence of two eventsof cement deposition characterized by differentlevels of kinetic energy (Fig. 6C).

The following unit (henceforth called ‘Unit 3’)comprises poorly sorted medium-grained fossil-iferous sandstone, with chaotic/diffuse bedding,reaching up to 1�5 m in thickness (see Figs 3and 4). It lies unconformably on the previousunit and infills the referred palaeo-channel; inone of the edges of the palaeo-channel, flamestructures, tails of entrained sediment from theprevious sequence, and a large intraclast (rip-upclast) are present (see Fig. 5C and D), suggestingstrong erosion and rapid settling. In a similarfashion, remains of rock-encrusting bryozoancolonies that were ripped from their hard-substrate are found ‘floating’ within the sand-stones (Bj€orn Berning, pers. comm.) – probablycorresponding to Ellisina sp., Onychocella sp.,Hemicyclopora sp. and Reteporella sp. – as wellas remains of isolated corals. Fossils and shelldebris are generally dispersed in random/chaoticpositions. It is not possible to confidently match

Fig. 4. Strip logs along the locations identified in Fig. 3. Labels #1–6 represent sampling location for thin sec-tions, of which pictures of the 4 most representative lithologies are shown in Fig. 6.



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Fig. 5. Photographs of detailed aspects of the outcrop, fossils and existing sedimentary structures. (A) View ofthe outcrop, showing the erosional unconformity between Unit 4a and an underlying crest comprising Units 1 to3. Location of strip log 1. Person for scale is ca 1�8 m tall. (B) Detail of the erosional contact between the massivetempestite Unit 4a (exhibiting preserved swales and hummocks) and the underlying sequence comprising Units 1to 3. Location of strip log 2. One Euro coin for scale, diameter 22 mm. (C) Detail of the erosional unconformitybetween Units 2 and 3, showing the palaeo-channel and large rip-up clast of Unit 2 floating in Unit 3. (D) Detailof one the sides of the palaeo-channel, showing flame and fluid escape structures of bioclastic-rich sedimentof Unit 2 onto Unit 3. The side of the rip-up clast is also visible in the upper right section of the photograph.(E) Contact between top of the sedimentary sequence and the overlying submarine lavas (pillow-lavas and hyalo-clastites). Hammer for scale ca 330 mm long. (F) Detail of imbrication of bivalvia shell in fossiliferous sandstone.(G) Examples of allochthonous fossil gastropods found at the outcrop: Alvania sleursi (top), Anachis avaroides(base) typical of shallow marine water conditions (common size 2 to 4 mm).



units observed along the western portion of theoutcrop with its eastern portion (see Figs 3 and4); however, it seems that the sediments at thebase of Log 3 may be etheropic with the above-described unit. If that is the case, there is alateral variation from chaotic/diffuse stratifica-tion in the western portion of the outcrop, to afaint cross-stratification towards the east. The pet-rographic analysis shows that the constituents arerepresented mostly by palagonite fragments and areduced amount of lithics (small rounded vesicu-lar pyroclasts and angular lava fragments), all ran-domly arranged and with a very heterogeneousgrain size (Fig. 6D). The presence of two sparitecement types is also confirmed here.The next event produced a profound erosive

unconformity that cuts through all the previousunits, forming an irregular palaeo-topography

with abrupt troughs, up to 3 to 4 m deep andabout as wide as they are deep (see Figs 3, 5Aand 5B). This palaeo-topography was, in turn,filled by a 4 to 5 m thick unit (henceforth called‘Unit 4a’) of fossiliferous, medium-grained sand-stones exhibiting a distinctive metre-scale hum-mocky structure. This stratification is generallycharacterized by 1 to 3 m wide, 0�3 to 1�5 mhigh slightly asymmetrical hummocks andswales that coarsely follow the underlyingtopography and exhibit onlapping contacts alongthe steep channel/trough sides; beds typicallyfan out from conformable almost-parallel finebeds, internal truncation surfaces are rare, andhummocks are almost ubiquitously preserved(see Figs 3 and 5B). The small extension ofthe outcrop and the lack of a proper three-dimensional view unfortunately preclude the

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Fig. 6. Photographs of thin sections, according to sample locations marked in Fig. 4. (A) Microphotograph show-ing the structure and lithological composition of a sample from Unit 1. Good sorting and a reduced amount of fos-sils and loose crystals characterize this sample. (B) Microphotograph of a medium-grained sandstone from Unit 2.A sharp reduction in abundance of glassy shards and the presence of palagonite clasts are the main characteristicsof this sample. (C) Two kinds of sparite cement are well evident in this picture of a sample of Unit 2. (D) Massive(unsorted) structure of a sandstone of Unit 3. Palagonite clasts are present in large amounts.



observation of a preferred dip direction in thethird dimension, i.e. making it very difficult toverify whether the existing stratification is iso-tropic or anisotropic. The sediments are gener-ally poorly sorted, although some layers ofslightly coarser grain size help to define the over-all bedding structure, and typically stand in posi-tive relief along the outcrop face as a result ofdifferential erosion. Sediments tend to be coarserand richer in shell debris at the bottom of thetroughs, or at the base of deeper swales. Gradu-ally, towards the top, bedding becomes almostplane-parallel, with occasional smaller-scale rip-ples. In the western portion of the outcrop, thesedimentary sequence is incomplete since it wastruncated atop by the later (MIS 5e) shore plat-form, located ca 4�5 m apsl and contemporarywith other known MIS 5e features found aroundthe island and at similar elevations (Callapez &Soares, 2000; �Avila et al., 2009). On the easternside, however, the sequence is complete and thetop can be observed, despite the poor accessibil-ity and more intense weathering. Fossil contentcomprises allochthonous components of a shal-low fauna. Specifically, the presence of extantmicrogastropods, such as Anachis avaroidesNordsieck, 1975 and Alvania sleursi Amati, 1987(living respectively at around 3 to 6 m and 10 to20 m apsl; �Avila, 2003) (Fig. 5G), together withthe shallow extinct balanid Zullobalanus san-tamariaensis and various sublittoral echinoids –for example, spines and fragments of Eucidaristribuloides (Lamarck, 1816), complete tests ofEchinoneus cf. cyclostomus Leske, 1778 andEchinocyamus pusillus (Muller, 1776) (Madeiraet al., 2011) – as well as rock-encrusting bryo-zoan colonies that were ripped from their rockysubstrate at intertidal to subtidal levels, onceagain confirms the energetic removal and trans-port of sediment and organisms from shallow lit-toral zones to deeper settings. This is alsosupported by the presence of other fossil debrisresulting from shored epipelagic species, such asJanthina typica (Br€onn, 1861), and typical littoralfaunas (e.g. Arca noae Linnaeus, 1758, Crassad-oma multistriata (Poli, 1795), Gari depressa(Pennant, 1777) [=Psammobia aequilateralis])that can be found mixed within the sediment.Complete fossil organisms and shell fragmentsare generally dispersed within the sediment andin random/chaotic positions; in fact, shells areoften found (single, disarticulated valves) innearly vertical positions. The remaining part ofthe sequence is only seen in the eastern portionof the outcrop.

Above the previous sediments (only visible inlog 3), and bounded by two faint erosive sur-faces of minor importance, a thin layer (ca0�5 m thick, henceforth called ‘Unit 4b’) of fos-sil-rich, medium to coarse-grained sandstone,with no apparent bedding structures occurs.This layer is very rich in microgastropods, bival-ves, fragments of Zullobalanus santamariaensisBuckeridge & Winkelmann, 2010 (Winkelmannet al., 2010), several bryozoan species, echinoidsand rhodoliths. Shell debris is randomly/chaoti-cally distributed within this layer, with no pref-erential geometric arrangement.Finally, the sedimentary sequence ends with a

massive sandstone unit (henceforth called ‘Unit4c’), amounting to ca 3�7 m in total thickness (seeFigs 3 and 4). Unit 4c is fossil-bearing and showsnormal graded bedding. The first ca 1�2 m corre-spond to medium-grained sandstones exhibitinga faint cross-stratification, and with some minorbioturbation (burrows) towards the top. Thesegrade from medium to fine-grained sandstones;the transition is conformable, gradual and onlymarked by the presence of bioturbation and thechange from faint cross-stratification to an almostimperceptible plane-parallel sparse stratification.Bioclastic debris decreases in abundance towardsthe top of the sequence where few sparse valves,showing some slight downslope imbrication,occur (see Fig. 5F). The uppermost ca 1�5 m ofthis sequence is intensely bioturbated by burrow-ing organisms (Diplocraterion isp., Thalassino-ides isp., Ophiomorpha isp., Rhizocorallium isp.and Crossopodia isp.), leading to destruction ofthe original sedimentary structures. Basaltic pil-low-lavas of the overlying lava delta cap thesequence, imprinting load casts in the once softsediments and showing without any doubt thatthe volcanic sequence is penecontemporaneouswith the underlying sediments (see Fig. 5E).

DISCUSSION

Sedimentological and palaeontologicalevidence

Marine sedimentary bodies exhibiting hummockyand swaley cross-stratification have been relatedto storm events that transport sediment under theinfluence of wave oscillation conditions and/orcombined wave oscillation and unidirectionalflows from coastlines onto the shelf (Myrow &Southard, 1996; Myrow, 2005; Dumas & Arnott,2006). On the other hand, turbidity-like deposits



are thought to be deposited below storm-wavebase because they lack the characteristics of thedeposits formed under bidirectional flows. In theopinion of the present authors, the sedimentarysequence at Ponta do Castelo represents a rapidsuccession of four or five different events, eachinvolving voluminous sediment transport fromshallow waters to greater depths, followed byrapid deposition by the action of storm-relatedunidirectional and/or combined flows:

Unit 1The first event is partially exposed because itsbase lies below present-day beach sediments. Itis inferred, however, that the observed stratifica-tion is part of a much larger structure of hum-mocks and swales, and thus interpreted as atempestite formed under wave oscillations (orcombined flows) above or close to storm-wavebase.

Unit 2The next event with turbidite-like features wasmost probably generated by a density-inducedflow that brought sediments from shallowerlevels (as suggested by the presence of alloch-thonous littoral fauna), initially eroding the seabed and eventually settling as it gradually lostenergy. In the opinion of the present authors,because the shelves in other Azorean islandscan be quite steep (2° to 3° on average, butlocally reaching higher values) (Quartau et al.,2010; Mitchell et al., 2012), it is not that pecu-liar to have storm-related turbidites. Thesequence probably is not complete, because itlacks the rippled facies that are normally foundatop similar sequences. It was probably erodedby the following event, as indicated by the pres-ence of the upper unconformity. Nevertheless, itwas not strong enough to totally remove the bio-turbated top of the sequence that shows that atleast a period of fair weather occurred betweenevents, allowing biological colonization of thetop of the deposit.

Unit 3The third event is sandy, rich in shell debris,some of it in random positions, and exhibitschaotic/diffuse bedding that eventually gradesinto cross-bedding. The sequence infilled thepalaeo-channel carved in the previous unit, andthe presence of rip-up clasts (intraclasts) sug-gests that erosion and sedimentation (under veryrapid aggradation rates) were violent and almostinstantaneous. The existence of well-preserved

bryozoan colonies that were ripped from theirshallow-water rocky substrate and that are nowmixed in the sediment, attests to the vigorousenergy of the event and the transport of shallow-water/littoral material to greater depths. Thisunit is thus interpreted as another tempestitedeposit, although the lack of distinctive sedi-mentary structures precludes unequivocal infe-rences concerning the type of mechanisminvolved in the deposition.

Unit 4In contrast, the massive deposit that follows –Unit 4a, representing the fourth event – isoutstanding and crucial to understanding of thesedimentary dynamics occurring here. Firstly,the deposit overlies a deeply irregular palaeo-topography – with 3 to 4 m deep troughs cuttingdeep into the previous sequences – attesting tothe vigorous erosion necessarily associated withvery strong sea floor currents. Secondly, but noless important, it exhibits a distinct mega-hum-mocky stratification, with preserved metric,slightly asymmetrical hummocks with rare inter-nal truncations, suggesting deposition underextremely high aggradation rates – perhaps cor-responding to a deposition time of just a fewhours – as a result of combined flow conditionsabove or at storm wave base (see Dumas & Ar-nott, 2006). Sedimentation under extremely highaggradation rates is also supported by the pres-ence of the following features: bedding coarselyparallel to the underlying topography; beds withonlapping contacts on the sides of the troughs/channels but with lateral continuity outside/above these depressions; the absence of internaltruncations inside the channels; the presence oftails of sediment eroded from the previoussequence and entrained in the infilling sedi-ments; and fossils chaotically distributed in ran-dom positions (including vertically). Similar toother units, it contains fossils that lived in shal-low littoral settings and were transported down-slope.Above the previous package lies a 0�5 m deb-

ris flow deposit (Unit 4b) that might have beenassociated with the same event or, less likely,occurred during a different event. The latterbrought more organisms/sediments from littoral(intertidal to sublittoral) zones, as its fossilcontent suggests.Finally, the last sedimentary unit – Unit 4c –

in the opinion of the present authors representsthe transition to the fair-weather suite. Thecross-stratification at the base still represents



wave oscillation influence but with a gradualpassage to less energetic conditions (as attestedto by the fining upwards graded bedding); theintense bioturbation present towards the top ofthe sequence is probably related to the veryrapid, opportunistic biological colonization thattypically occurs after a storm event, in suchrevolved ‘new’ bottom environments. The sedi-mentation cycle was subsequently interruptedby rapid progradation of a costal lava-fed deltasustained by an eruption on land, and the sedi-mentary structures/deposits preserved.In summary, the sedimentary features

described above suggest that the depositionoccurred under very high levels of sea floor flowenergy. The mega-hummocky stratification, theerosive unconformities, sedimentary structures,poor selection of grain size, deposition of elon-gated bioclasts in vertical position and localaccumulations of shells in chaotic positions areall clear evidence of storm-wave and storm-cur-rent imprint onto the sea floor (Kidwell &Bosence, 1991; Mount & Kidder, 1993; Myrow &Southard, 1996; Myrow, 2005; Dumas & Arnott,2006; Vierek, 2010). Therefore, the presentauthors believe that the sedimentary deposits atPonta do Castelo were created by a sequence ofstorm events, each one deposited in a singlestorm and partially eroded by subsequent events.

Present-day oceanographic andsedimentological evidence of tempestitedeposition in the Azorean insular shelves

The present authors believe that the full sedi-mentary sequence was deposited very rapidly,during a few tens or hundreds of years, and thatpreservation was only possible due to ongoingvolcanic activity. This assumption is fairlyconfident because present-day rates of sedimen-tation are very high in insular shelves of theAzores archipelago. Sedimentary depositsaround the insular shelf of Faial, which canreach up to 50 m in thickness, are believed tohave formed during the last 6�5 ka, based onestimations from coastal and subaerial erosionand explosive volcanism (Quartau et al., 2012).This implies modern sedimentation rates around8 mm per year, or even more, because there isevidence of sediments crossing the shelf breakto the slope of the island. Thus, if the wholetempestite sequence at Ponta do Castelo corre-sponds to the sedimentation of several season/storm events that occurred in a short time interval(due to the very short recurrence period of

storms in the Azores), as postulated herein, thenthe top of the deposit is penecontemporaneouswith its base and the palaeo-water depthinferred from the overlying volcanic structure(ca 45 m) could be extended to the underlyingtempestite deposit, with important implications;one is that storm-wave base in such environ-ments reaches down to depths of 50 to 60 m,and the other is that storm-induced downwel-ling currents are responsible for bringing largeamounts of littoral sediments to those depths.This is not surprising since the nearshore part ofthe insular shelves surrounding Faial and Picoislands are mostly rocky and sedimentary depo-sits develop mostly on the middle to outer shelf(Mitchell et al., 2012; Quartau et al., 2012), mostprobably transported by storm-induced down-welling currents as observed in other energeticenvironments (Field & Roy, 1984; Nittrouer &Wright, 1994; Hern�andez-Molina et al., 2000;Chiocci et al., 2004).The estimation of fair and storm-wave base in

the Azores shows that sediments can react towave action at water depths of 50 m and below.Wave base is defined as the water depth beyondwhich wave action ceases to stir the sedimentbed (Cowell et al., 1999). Conventionally, thislimit is taken to be where the water depth (hLS)is half the deep-water wavelength (L0) – Eq. 1 –which corresponds to the deep-water transitionto intermediate-water waves:

hLS ¼ L02

ð1Þ

The general expression for wavelength accordingto linear Airy wave theory is (Dean & Dalrymple,1991):

L ¼ gT2s

2ptanh

2pL

� �ð2Þ

and for the special case of deep-water waves,tanhð2p=LÞ ! 1; so that wave length simplybecomes:

L0 ¼ gT2s

2pð3Þ

where L0 is the deep-water wavelength, Ts is theperiod and g the acceleration due to gravity(9�81 m sec�2). The calculations are made usinga 14-year period (from 1989 to 2002) of wavehindcast data from Carvalho (2003). Averageand maximum peak wave period (Tp) are 9�9 sec



and 19�8 sec, respectively. By converting to aver-age and maximum significant wave period (Ts)using Tucker (1991) relations between Tp andTs, values of 8�3 sec and 16�5 sec, respectively,are obtained. By replacing these values in Eq. 3,it can be seen that average waves are able to re-mobilise sediments to ca 50 m water depth andthe strongest storm recorded in the consideredperiod is able to achieve this down to 213 m.Lastly, the analysis of a current meter

deployed at ca 20 m water depth during eightmonths in the southern coast of Faial Islandprovides evidence for extremely strong bottomcurrents. Most of these currents (80%) wererelated to storm surges (Youssef, 2005) and thehighest bottom velocity recorded was 2 m sec�1

during a period in which the significant waveheight reached 5�12 m. It is reasonable to expecteven higher bottom velocities because the maxi-mum significant wave height attained duringthe 14-year period analysed reached 16�7 m(Carvalho, 2003). The present authors areassuming, however, that despite the warmer cli-mate inferred for the Neogene, storm conditionsdid not differ considerably from modern ones.This assumption is supported by the field evi-dence at Ponta do Castelo that could only beformed by storms which are as strong or evenstronger than today.

Depositional model of sedimentation in theinsular shelves of ocean volcanic islands

Based on this study and the oceanographic andsedimentological characteristics of the Azores, itwas possible to develop a model of howsedimentation occurs in settings similar to these.Although alongshore sedimentation might occurduring storms, the prevailing one appears to bedirected offshore.Storms, in favourable conditions (for example,

depending on wind direction to the shorelineand morphology of the coastline and shelf), maycause coastal setup and develop storm surgeebb-currents normal to the shore. In the case ofthe shelf of Faial, ca 50% of the currents mea-sured had angles greater than 45° to the shore(Youssef, 2005). Moreover, the data showed apositive relation between wave height and stormsurge current speed. The balance of forcesinvolved during storms can explain this correla-tion. According to Myrow & Southard (1996),offshore transport is aided by high-pressure gra-dient, high excess weight and low Coriolis force(or stronger friction force). High-pressure gradi-

ent is easily attained during big storms in theAzores because storm surges produce highcoastal setups which need to be compensated byopposite strong ebb or downwelling currents.On the other hand, turbulence added by highwaves maintains particles in suspension creatinghigh excess weight. The magnitude of this forceis a function of the downslope component of theexcess weight (per unit volume) of a sediment-rich dispersion relative to the clear water and ofthe bottom gradient. Therefore, offshore gravitydriven transport to the middle and outer insularshelf is promoted by strong ebb-currents (high-pressure gradient) and high excess weight(high concentrations of suspended sediment andhigh shelf gradients). The Coriolis force wouldnormally deflect these currents towards thealongshore direction; however, the sum of thethree forces plus the high friction force (bottomfriction, acting opposite to the direction of thecurrents) generated during the event, attenuatesCoriolis force allowing currents to maintain highangles to the shore (Fig. 7A; Duke, 1990; Myrow& Southard, 1996). Therefore, in shallow waters(above storm-wave base), strong oscillatory fluidmotion coupled with unidirectional currentstransport sediment from the nearshore and formhummocky cross-stratifications and wave rippleswhilst at greater depths (below storm-wavebase),the unidirectional flows dominate and turbiditicsequences form. During the ensuing fair-weatherconditions (Fig. 7B), the new sea-bottom sedi-mentary cover is rapidly colonized by endobenth-onic organisms producing bioturbation of theupper part of the deposit. The present authorsbelieve that this study may help to resolve theconflict between the common oceanographicalobservations of shore-parallel geostrophic stormflows and the geological observations which sug-gest mostly cross-shelf transport mechanisms.Most researchers believe that the mobilization oflarge quantities of sand towards offshore wouldhappen very rarely, with recurrence intervals oftens of thousands of years (Myrow & Southard,1996). However, according to Tsutsui et al.(1987), hurricane Iwa in Oahu, Hawaii, that pro-duced significant wave heights of ca 5 m, a max-imum wave height of ca 9 m and sustainedwinds of 30 to 35 knots, was able to transportcoarse sands and gravels from sediment bodieslocated on the shallow shelf onto the slopes ofthe island. In the Azores, the return period ofmaximum wave heights of 12 m is five years(Carvalho, 2003), which supports the claimherein of high recurrence storm deposition.



AB

Fig.7.Cross-insu

larsh

elf

profile:(A

)Storm

conditions.

Storm

coastalsetup

isbalanced

bystrongdownwellingsh

ore

obliquecurrents

‘DC’.These

resu

ltfrom

thesu

m(redarrow)ofstronghigh-pressure

gradient,excess

weight‘P/E’,strongfrictionforce‘F’andlow

Coriolisforce‘C’(seetextforexplanation).

(B)Fair-w

eatherconditions.

Nearshore

sedim

entwastransp

orted

basinward

and

deposited

below

50m

waterdepth.Aboveand

below

storm

-wavebase,

resp

ectively,hummockycross-stratificationsandturbiditedeposits

are

found.



Therefore, the studied outcrop, coupled withcurrent knowledge of the present-day sedimen-tary processes in these islands, shows that inwave to storm-dominated island shelves, asthese are, it is possible to have significant cross-shelf transport of sediments and that storms arethe main process of deposition in these settings.The present authors believe that these conclu-sions can be extended to a much wider range ofenvironments, such as those on narrow andsteep continental shelves with a suitable waveenergy regime. Modelling (Cookman & Flemings,2001; Quiquerez et al., 2004) and observations(Gagan et al., 1990; Cacchione et al., 1994;Amos et al., 1996; Li et al., 1997) suggest thatwinter storms are the main agent causing sedi-ment dispersal on the worldwide continentalshelves. Therefore, on narrow, steep and waveto storm-dominated shelves, cross-shore sedi-ment transport is apparently the main mecha-nism of deposition (e.g. Field & Roy, 1984;Chiocci et al., 2004). This model could even beapplied to wide continental shelves during low-stands of sea-level.

CONCLUSIONS

This study, combined with the current know-ledge of the modern oceanographic and sedi-mentological characteristics of the Azoreanshelves provided important clues to reveal themain processes responsible for the transport anddeposition of sediments around open-sea volca-nic islands:

1 The sedimentary deposits of the sequence atPonta do Castelo are interpreted as tempestites,and thus storms were the main agent of trans-port and deposition on the insular shelf of SantaMaria during the Neogene.2 The described sedimentary sequence was

probably deposited very rapidly, during decadesor centuries and was only preserved because itwas covered by a lava delta, hence protected,and subsequently exposed subaerially by upliftand erosion.3 The inferred rapid deposition of the sedimen-

tary sequence and the lava delta on top of it wereused to estimate very accurately the water depth atwhich the deposition occurred, ca 50 m, which isnot that peculiar given the modern examples inthe literature (e.g. Tsutsui et al., 1987).4 Storms are very frequent in the Azores, with

very high waves and strong winds that produce

storm surges. High-pressure gradients againstthe coastline are balanced by offshore-directedebb-currents. Thus, nearshore sediments are eas-ily entrained and maintained in suspension bystorm-waves and currents. The combination ofhigh excess weight of the suspended sediments,relatively high gradients of the shelf and thestrong-storm surge ebb-currents provide themechanism to transport sandy sediments downto depths of 50 m and more.5 The present authors claim that the main pro-cesses of transport and deposition on insularshelves are strong ebb-return currents that areformed during storm events. The hummockystratification and ripples found within thesequence are deposited above storm-wave baseand the turbidite-like beds are deposited belowstorm-wave base.

The authors hope that this study will increasethe debate on the mechanisms of tempestitedeposition. Further research on other upliftedvolcanic islands is needed to contribute to a bet-ter understanding of the sedimentary processesacting on the insular shelves of these islands.The authors also consider that this model couldbe applied to other environments, such as nar-row and steep continental shelves or even towide continental shelves during sea-level low-stands, as long as remarkable wave energy ispresent.

ACKNOWLEDGEMENTS

The following agencies are gratefully acknow-ledged for providing funding: Direccao Regionalda Ciencia, Tecnologia e Comunicac~oes (DRCT)of the Regional Government of the Azores andFundacao para a Ciencia e a Tecnologia (FCT) ofthe Portuguese Government; field assistance:Clube Naval de Santa Maria and Camara Muni-cipal de Vila do Porto; and GIS topographicmapping of Santa Maria Island: DRTC andDireccao de Servicos de Cartografia e InformacaoGeogr�afica. We are also grateful to the organizersand participants of several editions of the Inter-national Workshop ‘Palaeontology in AtlanticIslands’ who helped in the fieldwork. We greatlyappreciated helpful comments from StephenRice (Editor), Jess Trofimovs (Associate Editor),Brian Jones and an anonymous reviewer. DFG –The German Research Foundation providedRicardo Ramalho’s funding, through the project‘Investigation of ocean island uplift in the



Azores Island Region’. Vittorio Zanon was fundedby DRCT grant M3�1�7/F/008A/2009. Rui Quartauand Sergio �Avila acknowledge their Ciencia 2008research contracts funded by FCT.Ana CristinaRebelo and Ricardo Piazza Meireles benefited,respectively, from FCT grants SFRH/BD/77 310/2011 and SFRH/BD/60 518/2009.

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Manuscript received 6 November 2012; revisionaccepted 17 April 2013



Depositional processes on oceanic island shelves–Evidence from storm‐generated Neogene deposits...

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