REVIEW ARTICLE

New insights in the evolution of Antarctic glaciation from depthconversion of well-log calibrated seismic section of Prydz Bay

V. Volpi Æ M. Rebesco Æ P. Diviacco

Received: 31 January 2007 / Accepted: 6 July 2008 / Published online: 26 July 2008

� Springer-Verlag 2008

Abstract The understanding of the evolution of the

Antarctic Ice Sheet is crucial for the comprehension of the

history of past global climate. The debate regarding the age

of the transition to modern ‘‘dry- and cold-based’’ ice sheet

after the Neogene polythermal conditions has taken place

over more than 20 years. An evident change in the

geometry of the depositional systems of the Prydz Bay

continental margin demarked the initiation of the Prydz

Channel Fan and has been inferred to correspond to this

transition. The improvement in the age placement of this

change contributes to unravel the last stages of the Ant-

arctic glacial history. We predicted the spatial distribution

of P-wave velocity data along both dip- and strike-oriented

seismic profiles that intersect 3 Ocean Drilling Program

(ODP) sites on the Prydz Bay continental shelf. We used

this information to assist the correlation of the existing

litho- and bio-stratigraphic information among the drilling

sites and to produce an accurate geometric reconstruction

of the Neogene shelf units through depth-migration of the

seismic data. The revised stratigraphy that we obtained

suggests an early late to late early Pliocene age for the

seismic reflector at the base of the Prydz Channel Fan. This

age, younger than previously proposed, is consistent with

the age inferred for similar geometric changes identified in

different Antarctic margins.

Keywords Prydz Bay � Log property �Seismic attributes � Neogene stratigraphy

Introduction

The East Antarctic Ice Sheet is the longest lived and largest

ice mass on Earth (Barron et al. 1991). It has played a

central role in global climate, sea level change and ocean

and atmospheric circulation (Cooper and O’Brien 2004).

Its study complements the history of the long-term

global cooling recorded in the ocean d18O records.

On land and offshore data demonstrate that the ice sheet

was subjected to major fluctuations during polythermal

glacial regime in the Neogene, before the transition to the

modern glacial regime, which is usually referred to as

‘‘polar’’. However, recent evidence for possible intercon-

nection of modern Antarctic subglacial lakes suggests a

relatively developed hydrology beneath the present ice

sheet (Bingham and Siegert 2007). Nevertheless, this

hydrology is supposedly much less than that existing dur-

ing the short deglacial periods (Dowdeswell and Elverhøi

2002; Stokes and Clark 2001) and during the polythermal

glacial regime. For simplicity we will refer hereafter to

modern, polar regime as relatively cold- and dry-based

with respect to previous polythermal conditions.

A primary objective of offshore drilling in the Prydz

Bay region, which is considered a drainage area for the

earliest Cenozoic ice sheets, was to decipher the Late

Eocene onset of glaciation (Barron et al. 1991; Cooper and

O’Brien 2004). Nevertheless, drilling provided also evi-

dence of the Neogene ice sheet fluctuations, which

repeatedly eroded the continental shelf removing large

parts of the sedimentary record (Barrett 1996; O’Brien

et al. 2004).

In the Neogene the depositional systems of the conti-

nental margin show a dramatic change: shelf and slope

progradation increased and focused in the middle of Prydz

Bay in contrast with the earlier even distribution along the

V. Volpi (&) � M. Rebesco � P. Diviacco

Istituto Nazionale di Oceanografia e di Geofisica Sperimentale,

OGS, Borgo Grotta Gigante 42/c, 34010 Sgonico (Trieste), Italy

e-mail: vvolpi@ogs.trieste.it

URL: http://www.ogs.trieste.it

123

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

DOI 10.1007/s00531-008-0356-6

shelf edge (Cooper and O’Brien 2001; Cooper and O’Brien

2004). In correspondence with a progressive landward shift

of the depocenters, from the continental rise towards the

continental slope and base of slope (O’Brien et al. 2004),

sedimentation rates progressively declined on the conti-

nental rise. These changes are inferred by Cooper and

O’Brien (2004) to correspond to the transition to the polar

ice sheet. Prior to the change in fact, temperate glaciers

eroded onshore and intermittently on the shelf, resulting in

widespread erosion of a normal water depth shelf during

lowered sea levels. After the change, glacial maxima gla-

ciers in the Lambert graben merged into a major axial ice

stream that extended far onto the continental shelf, pro-

ducing localized erosion in deep trough areas.

However, the age of the final transition to the polar ice

sheet was not entirely resolved by the stratigraphic studies

conducted for the Neogene continental shelf in the frame of

ODP Leg 188 (Erohina et al. 2004; O’Brien et al. 2004;

Handwerger et al. 2004; Whitehead and Bohaty 2003). This

age has been inferred progressively younger: from the

middle-late Miocene (Cooper and O’Brien 2001), to the

middle Miocene–Pleistocene (Cooper and O’Brien 2004), to

the late Miocene or early Pliocene (Whitehead et al. 2006).

On land evidence of Neogene ice sheet fluctuations is

provided by the Sirius Group, whose age has been under

intense debate since over 20 years. The so-called ‘‘Stabi-

lists’’ have argued that the Sirius Group predates the Dry

Valleys landscape, which is inferred to has remained stable

at least since 14 Ma, i.e. since middle Miocene (Marchant

et al. 1993, 1996; Sugden et al. 1993; Sugden 1996; Stro-

even et al. 1998; Stroeven and Kleman 1999). Conversely,

the so-called ‘‘Dynamicists’’ have argued that the ice-sheet

fluctuations, which are indicative of more temperate

(polythermal) glacial regimes than the present one, lasted

until about 2.5 Ma, i.e. the late Pliocene (Webb et al. 1984;

McKelvey et al. 2001; Wilson 1995; Harwood and Webb

1998; Hambrey and McKelvey 2000; Hambrey et al. 2003).

Whitehead et al. 2006 reviewed the Cenozoic stratigraphy

of the whole Lambert Graben and Prydz Bay region pro-

viding a general overview of temporal and spatial history

of ice sheet advance and retreat.

The aim of this work is to further contribute to the

constraining of the transition to the polar ice sheet by

producing an integrated stratigraphy of the glacial

sequences of the continental shelf in the Prydz Bay area.

For this purpose we employ an innovative technique to

predict, along seismic profiles, the distribution of log

properties (i.e. P-wave velocity) measured at three sites

of ODP Leg 188 and 119. The resulting distribution of

P-wave velocity is then used to correlate the existing lith-

ologic and biostratigraphic information. Furthermore, it is

used to produce an accurate depth-converted seismic pro-

file, which illustrates the correct geometric relationship of

the glacial sequences of the continental shelf in the Prydz

Bay area.

Geological setting

Prydz Bay is roughly triangular in outline with an east-west

baseline at the continental shelf edge and a southward

pointing apex. The southwest margin is formed by the

Amery Ice Shelf while the southeast margin of the bay is

formed by the rugged and generally north-east trending

Ingrid Christensen Coast (Stagg 1985). Prydz Bay lies at

the oceanward end of a graben, which is occupied by the

Lambert Glacier and Amery Ice Shelf (Fig. 1).

Most features of the shelf and upper slope were pro-

duced by the advance and retreat of the Lambert Glacier

and Amery Ice Shelf since late Eocene to Oligocene times

(Hambrey et al. 1991; Cooper et al. 1991a).

As the other Antarctic continental shelves, Prydz Bay

shows a number of inner shelf deeps and outer shelf banks;

it is occupied by a broad topographic basin, the Amery

Depression, which deepens landward, and by a relatively

deep trough, the Prydz Channel, that crosses the outer shelf

on the western side of the bay.

The present ice-drainage basin is believed to be long-

lived (Barron et al. 1991; Cooper and O’Brien 2004) and to

have responded to mass balance fluctuations in the interior

of the East Antarctic Ice Sheet. Leitchenkov et al. (1994)

analyzing the shelf edge progradation recognized three

main post-Oligocene events in the advance-retreat history

on the Prydz Bay shelf. They have been interpreted in

terms of till-tongue stratigraphy based on model of King

et al. (1991).

The stages of Antarctic glaciation and the pre-glacial

continental climate are inferred to be recorded in the sed-

iments of Prydz Bay (Cooper and O’Brien 2004).

During the late Neogene, the Lambert Glacier-Amery

Ice Shelf drainage system flowed across Prydz Bay in a

fast-flowing ice stream that cut Prydz Channel and

deposited a large amount of debris on the upper slope

(O’Brien et al. 2004). O’Brien and Harris (1996) inferred

that the Prydz Channel and Fan started to develop in the

early to mid-Pliocene, when the amount of ice accumu-

lating in Princess Elizabeth Land at the southeastern side of

Prydz Bay increased to the point where it deflected the flow

of the main Lambert–Amery system westward. Different

glacially formed features testify the complex glacial history

of the area. O’Brien and Harris (1996) argued that erosion

and deposition by the Lambert Glacier in Prydz Bay fell

into three zones: the inner zone where inner shelf deeps

formed by enhanced erosion in areas of thick ice due to the

maximum basal shear stress; the middle zone, underlain

by transitional marine sediments, deposited during retreat,

1992 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

both as blanket-like deposits and as grounding zone mor-

aines; the outer zone with deposition of subglacial till and

prograding continental slope deposits during full glacial

conditions.

Acoustic and physical characteristic of the seismic units

Stagg 1985 defined several acoustic units in the Prydz Bay

area. These seismic units were named from the shallowest

(PS.1) to the deepest (PS.6); units PS.1 to PS.4 are of

sedimentary type, whereas units PS.5 and PS.6 represent

the acoustic basement. Cooper et al. 1991b, based on data

acquired during ODP Leg 119, inferred that the topmost

units PS.1 and PS.2A (late Eocene/Early Oligocene-

Holocene) are constituted by glacial deposits, topset beds

and prograding sequences, respectively, deposited in front

of a grounding ice sheet. The underlying sequences are

composed by preglacial deposits (PS.2B–PS.4), by meta-

morphic rocks (PS.5) and igneous intrusive rocks (PS.6).

Many of the units identified in the area cannot be correlated

between the drilling sites, except for seismic unit PS.1,

since not all these units extend throughout the shelf

(Fig. 2).

Seismic unit PS.1 has been detected at all drill sites in

the Prydz Bay area and it extends between Neogene/

Paleogene unconformity, represented by the top of unit

PS.2A, and the seafloor (Fig. 2). At all drill sites on the

Fig. 1 Location map of the

Legs 119 and 188 related sites,

seismic profiles used in the

present paper. A–A0 and B–B0

are the two cross-sections along

the Prydz Bay area presented in

Fig. 2

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 1993

123

continental shelf this unit comprises only topset beds.

Nevertheless, Fig. 3 shows that the internal reflectors are

not simply, showing a subparallel and more or less laterally

continuous character. On a smaller scale, other glacially

formed features were mapped in the inner-and mid-shelf

area. Most of the features are a result of recent endogenous

and possibly neotectonic processes. Their morphological

expressions are asymmetric ridges with a steep western

side and a more gently dipping eastern side. The character

and location of the ridges suggest an origin as grounding

line moraines (Leitchenkov et al. 1994).The base of the

unit is generally a continuous reflector, but locally the

signal is disturbed by the complex reflectors of the over-

lying sequence (Figs. 4, 5, 6).

The base of unit PS.1 is represented by a truncation

representing a period of erosion where hundreds meters of

sediments were removed at Site 742 (Solheim et al. 1991)

and at Site 739 (Forsberg et al. 2001) (Figs. 4, 5, 6). The

underlying unit PS.2A has been divided in three reflection

packages (Erohina et al. 2004): the lower one fills the

depressions of the undulating surface at the top of unit

PS.2B; the intermediate one shows homogeneous and low

amplitude reflections; the upper one shows high-amplitude

reflections that are truncated at the overlying unconformity

(Fig. 3). On the dip profiles, it is a wedge-shaped unit

overlying the gently dipping reflectors of the preglacial unit

(PS.2B), eroded and deformed by the grounded ice

(Figs. 4, 5, 6). ODP data suggest that it was deposited by

the grounded ice sheet that advanced for the first time onto

the shelf in late Eocene to early Oligocene time.

Methods

In this work we analyzed seismic data and physical prop-

erties around three drill sites (739, 742 and 1166) located

on the continental shelf of Prydz Bay. Sites 739 and 742

were drilled during Ocean Drilling Program (ODP) Leg

119 while Site 1166 during ODP Leg 188. Site 739 is the

most seaward of the three sites while Site 742 is located

29 km southeast (Fig. 1).

Seismic data

We focused our analysis onto two seismic profiles: the

single-channel line Palmer 01-1-4 (hereafter Palmer04)

(Figs. 1, 3), that is almost perpendicular to the Prydz

Channel and crosses through ODP Sites 1166 and 742. It is

a high resolution single-channel line, collected by the R/V

Palmer in 2001 (Handwerger et al. 2004; Erohina et al.

2004). Multichannel line PB33-21 is subparallel to the

Prydz Channel and crosses Sites 742 and 739 (Figs. 1, 4)

collected and processed by the Bureau of Mineral

Resources (BMR), described firstly by Stagg (1985). Then

it was reprocessed for the strong reverberations as pre-

sented by Cooper et al. 1991b.The drilling depth is

maximum at Site 739 (450 m of logging record), but our

analysis focused within the upper 300 m of the strati-

graphic record, the depth reached by logging at all sites

location. On seismics this is equivalent to seismic unit PS.1

and the upper part of unit PS.2, following the nomenclature

proposed by Stagg 1985.

Fig. 2 A–A0 and A–A0 are cross-

sections across the Leg 188 drill

sites and Leg 119 drill sites,

respectively. They were

compiled on the available

seismic profiles of Prydz Bay

seismostratigraphy, from

Cooper and O’Brien 2004. See

location in Fig. 1

1994 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

Other seismic lines were interpreted in this work: Line

PB33-23 (subparallel to and further west of the PB33-21,

crossing only Site 739, Figs. 1 and 5), and Line 32002,

acquired within the 32nd and 33rd Soviet Antarctic

Research Expedition, from 1986 to 1988 (Kuvaas and

Leitchenkov 1992). As part of this work, the field data of

this seismic profile, courtesy by German Leitchenkov, were

stacked (24-fold stack of common depth-point gathers

spaced 25 m apart) (Fig. 6).

The data processing includes the application of the

‘‘amplitude recovery technique’’ which is crucial for the

following phases of the analysis. Seismic and log correla-

tion is very sensitive to the amplitude of the signal so

whenever possible the data should be processed taking into

account this peculiarity. In this work raw seismic data were

available only for the Line 32002. The method applied

consists firstly in determining an arbitrary exponential gain

function, with the only objective of having signals that can

Fig. 3 Single trace seismic profile Palmer04 (shot 2400–5300)

perpendicular to the Prydz Channel (from Erohina et al. 2004)

crossing Sites 742 and 1166, with the interpreted version. In the close

up it is visible the very continuous reflector close to the base of PS.1

that actually lies above the PS.1 base sampled in Site 742

Fig. 4 BMR PB33-21

multichannel seismic profile,

sub-parallel to the Prydz

Channel crossing Sites 739 and

742. See the text for further

details on the seismic data. The

seismic stratigraphy is from

Cooper et al. 1991

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 1995

123

be used in the following velocity analysis. An initial

velocity model is calculated and the amplitude loss due to

spherical divergence is corrected. The data are then sub-

jected to the processing steps till a final model of velocity

is obtained. This velocity model is used to calculate the

exact amplitude correction. At this point the data are pro-

cessed obtaining the final stack section.

Traces have been deconvolved before stack. Accurate

velocity analyses were conducted and the resulting velocity

functions were applied in the F–k filtering (multiple

attenuation) before the final stacking.

Well information

The physical property analyzed in this work is the P-wave

velocity. It was chosen because it is the only property that

was measured at all drill sites. The most reliable data are

provided by downhole logging. They provide continuous

and rather uniform velocity profiles from the top to the

bottom of the three holes. After a carefully editing phase,

we verified that, except for some occasional spikes, the log

is not particularly affected by noise or other effects due to

the conditions of the hole as also reported in the Shipboard

Scientific Party (1989d, 2001a, b) site related chapters.

In particular, P-wave velocity log measured at Site 1166

spans from 30 to 367 mbsf (meters below seafloor). The

data were acquired using a DSI (Dipole Shear Imagery)

sonic tool (Shipboard Scientific Party 2001a, b).

Sonic log velocity measured at Sites 739 and 742 were

obtained using the LLS (Long Spaced Sonic) tool (Ship-

board Scientific Party 1989a).

The first 30 mbsf were not measured by the logging

instruments so we integrated the velocity curve with core

data. To do this, we employed an iterative process in which

we initially assumed the correspondence between coring

and logging depths. Subsequently, a synthetic seismic trace

has been generated using P-wave log data and a constant

density of 1 g/cm3. The assumption of a constant density

Fig. 5 BMR PB33-23 multichannel seismic profile, sub-parallel to the Prydz Channel, oriented NE–SW. The location of Site 739 lies with

3.75 km offset from the line and it was projected on the seismic profile

Fig. 6 Multichannel line

32002, collected during a Soviet

Antarctic Expedition. The line

is oriented NW–SE, 50 m away

from Site 739. The data was

processed as part of this work.

Details are provided in the text

1996 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

value has been necessary since core density measurements

are unreliable at Sites 742 and 1166 due to bad hole con-

ditions or not recorded at all as at Site 739 (Shipboard

Scientific Party 2001a, b, 1989a, b). The synthetic seismic

trace was then compared with a trace resulting from the

average of three seismic traces extracted around the well

locations. A time-depth function has hence been calculated

to maximize the correlation between the two traces.

Finally, such function has been used to re-locate and

re-calibrate the logs in time.

Multi-attribute analysis

The prediction of petrophysical parameters (P-wave

velocity in the case of this work) along seismic profiles

(BMR 33-21 and Palmer04) has been performed using a

commercial software (Hampson–Russell EMERGETM

software package, Calgary, Canada). The program was

designed principally for use in hydrocarbon basins with

good acoustic continuity, unlike the examined Antarctic

shelf sections. For this reason we considered the resulting

prediction in a qualitative and not in a quantitative way.

Besides well-logs, that we employed in this work, core logs

or laboratory measurements on core samples may normally

be used to extract the petrophysical parameters that serve

as input for the prediction. To tie the seismic data to well-

logs we manually calibrated the logs to the seismic traces.

The technique used for this point is of wavelet extraction

and correlation. We extracted the wavelet from the seismic

data using e-Log program (Hampson–Russel). The internal

algorithm operates by finding the operator which, when

convolved with the reflectivity from the wells, closely

approximates the nearby seismic traces. The result was a

150 ms mixed phase wavelet with a taper length of 20 ms.

A stretch was then applied to the logs in order to maximize

the correlation between the seismic trace and the velocity

curve. The result of this fine stretching procedure yielded

to the generation of the adopted time–depth relationship. A

statistical correlation was then calculated among time

series of the petrophysical parameter (P-wave velocity) and

a number of time series of seismic attributes, extracted

from the seismic profiles crossing the boreholes. By

applying the best-fit correlation, a 2D section of the pre-

dicted distribution of the petrophysical parameter along the

seismic profile was obtained (Fig. 7a, b). The outline of the

procedure is the following: (1) a representative seismic

Fig. 7 Predicted spatial distributions of P-wave velocity obtained

extrapolating the velocity log measured at Sites 739 and 742 along the

tie seismic profile PB33-21 (a) and at Sites 742 and 1166 along the

Palmer04 profile (b). The two profiles are perpendicular one each

other allowing a 2D image of the lateral velocity variation along the

investigated area of the Prydz Bay shelf

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 1997

123

trace at each well location is obtained averaging a few

traces near the well; (2) starting from the trace, a set of

seismic attributes is derived, according to Taner et al.

(1979), considering the conventional seismic trace as the

real component of a complex trace. The basic attributes are

instantaneous amplitude, instantaneous phase, and instan-

taneous frequency. However, many additional attributes

can be derived from these (Barnes 1998). (3) A linear

regression is performed between pairs of seismic attributes

and petrophysical parameters in order to find a linear

relationship:

y ¼ aþ bx

where x is the independent variable (target petrophysical

parameters), y is the dependent variable (seismic attribute);

a and b are two constants determined by the regression. In

multiple-attribute analysis the target petrophysical param-

eter is modeled as a linear combination of several seismic

attributes. In the equation above, all terms are replaced by

matrixes. Because the linear relationships are often not

satisfactory, polynomial relationships with progressively

higher order can be used. (4) Different polynomials are

computed for an increasing number of seismic attributes.

(5) For each computed polynomials, the prediction error is

calculated as the root-mean square difference between the

actual value (that is the target log property) and the pre-

dicted value. (6) A ranking of polynomials is produced

according to decreasing errors, and a choice is made of

which number of attributes most significantly predicts a

given petrophysical parameter. (7) The chosen relationship

is then used to invert the seismic signal and predict the

spatial distribution of the petrophysical parameter along the

entire seismic section.

Seismic attributes analysis characterizes the internal

structure of the sediments in terms of physical properties,

with more details than what can be obtained by conven-

tional interpretation of seismic profiles (amplitude plot).

With multiple-attribute analysis, at each time sample of the

seismic profile the target log is modeled as a linear com-

bination of several attributes (available within the Emerge

software). The aim of this method is to use the seismic data

to predict the theoretical log at every CDP location in the

seismic profile. To do this we summed three traces around

the well locations and found a relationship between the

seismics at those locations and the measured logs. The

derived relationship is assumed to be valid for every trace

of the section and is consequently applied to the entire

dataset. We used two wells for the two seismic profiles:

wells 739–742 for line PB33-21, wells 742 and 1166 for

line Palmer04. We specified a time window for each log in

order to focus the prediction only where the data are con-

strained by the log measurements that is from the seafloor

to 1,000 ms (TWT). The average prediction error and the

number of attributes used for the prediction are shown in

Table 1.

Results

P-wave velocity distribution

From Site 742 to Site 739:

The overall distribution of P-wave velocity along line

PB33-21 shows that values are generally higher around Site

742 than around Site 739, in other words there is a ten-

dency of decreasing velocity moving seaward (Fig. 7a).

The upper part of the section (to about 680 ms depth)

shows an alternation of interfingering low velocity layers,

with values ranging from *1.9 to *2.2 km/s around Site

739 and from *2 to *2.3 km/s around Site 742. A sig-

nificant variation in velocity occurs deepward and has a

correspondence in the logs of both drill-sites. This anomaly

is a thin low velocity layer with values of *1.9 km/s at

*690 ms around Site 739 and of *2.0 km/s at *700 ms

around Site 742. Just below, a laterally continuous high

velocity unit characterized by values of *2.5 km/s extends

down to *735 ms. Further below, the seaward dipping

character of the reflectors results in different sections

drilled at the two sites, without direct correlation. The

velocity ranges from about *1.9 to *2.5 km/s and over,

broadly downward increasing, but with some minor

inversions in the trend.

From Site 742 to Site 1166:

The overall distribution of P-wave velocity along line

Palmer04 shows that values are generally similar at both

Site 742 and 1166, or slightly higher in the latter, which is

closer to the Prydz Channel (Fig. 7b). The upper part of the

section (to about 680 ms in Site 742 and 740 ms in Site

1166) shows a generally downward increasing trend of the

velocity values, from *2.0 to *2.4 km/s. At about

110 ms below the seafloor in both drill-sites, a significant

decrease corresponds to a continuous low velocity layer

with values of *2.0 km/s. Below this, the velocity values

increase to *2.5 km/s. However, this high velocity layer is

much thicker in Site 742 (from about 700 to about 735 ms

Table 1 Number of seismic attributes considered in the multi-attri-

bute analysis (see text for details) and the average error calculated at

each well location between the predicted and the original logs

Dataset Seismic attributes Prediction error (km/s)

PB 33-21 4 0.3

Palmer04 5 1.4

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123

depth) with respect to Site 1166 (from about 780 to about

795 ms depth). The upper part of this layer is missing in

the site closer to the Prydz Channel and apparently eroded

by the overlying unconformity. Below this layer, a low

velocity unit characterized by values between *1.9 and

*2.3 km/s is also thicker at Site 742 (between about 735

and 815 ms) with respect to Site 1166 (between about 780

and 820 ms). Velocity values in excess of *2.5 km/s are

found below 820 ms in both sites.

Depth conversion

The P-wave velocity field calculated from the seismic

attribute analysis for seismic profile PB33-21 has been used

to perform the depth migration (Fig. 8). We used the

velocity field obtained from the seismic attribute analysis

because it is very accurate providing detailed information

on the lateral velocity variation between wells along the

seismic line. Moreover, the logging measurements of

P-wave velocity represent a realistic velocity model for

depth migration Generally, migration is a process that

locates reflectors in their sub-surface correct position

allowing a true reconstruction of the seismic stratigraphy

along the considered profile.

Depth migration was performed on seismic line BMR

PB33-21 availing of the Paradigm GEODEPTH software

using the velocity model of Fig. 7a.

The velocity model produced by the EMERGE Hampson–

Russel software needed some pre-processing adjustments in

order to make it amenable to the migration process. The

problem encountered comes from the fact that EMERGE

does not provide velocity values along the water column

where obviously no log measurements exist. Therefore,

a constant value of 1,500 m/s was assigned to the water

layer.

On seismic line PB 33-21 the seafloor morphology

presents a ridge with a relief of about 30 m around Shot

2150. It is a structure that has its top at about 420 m with a

NNW steep side. The same structure was recognized along

other seismic profiles crossing the area (Figs. 4, 5, 6). In

correspondence of this ridge, there are three velocity

changes which have a correspondence in the time section:

(1) the presence of the ridge itself results in higher velocity

(with respect to the area just NNW of it, where only water

is present); (2) the interfingering of low velocity layers

occurs in the uppermost strata (Fig. 7a); (3) a change in

thickness of the high velocity unit at about 700 ms depth,

which is thicker NNW of the ridge. Migration took into

account these changes in reconstructing the geometric

relationships of the seismostratigraphic boundaries.

The migrated profile (Fig. 8) provides an image that is

clearer than the time section. In particular, this image

precisely illustrates the ‘‘three reflections’’ set at the base of

Unit PS.1. This set includes, from the base of PS.1

upwards: (1) a gently SSE-dipping negative-phase (white)

reflection at about 160 mbsf at Site 739 (purple reflector in

Fig. 8), which is terminated by erosional truncation in

correspondence of the ridge, without reaching Site 739; (2)

a negative-phase (white), concave reflection (red reflector

in Fig. 8), continuous between Sites 739 (about 130 mbsf)

and 742, but with varying amplitude as a consequence of

the tuning effect (Widess 1973; Sheriff 1977) produced

by the interaction with the adjacent reflections; 3) a

continuous, concave negative-phase (white) reflector

Fig. 8 Depth migrated version

of seismic profile PB33-21

across Sites 742 and 739. The

velocity field used for the depth

migration is the one obtained by

the multi-attribute analysis,

shown in Fig. 7a

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 1999

123

(green reflector in Fig. 8) at about 115 mbsf (clearly above

the base of unit PS.1 sampled in Site 742 (Fig. 3), which

tops the underlying very continuous positive (black) phase.

Integrated stratigraphy

The scientific results of ODP Leg 119 contributed sig-

nificantly to the definition of Prydz Bay stratigraphy,

analyzing seismic stratigraphy (Cooper et al. 1991a),

lithofacies (Hambrey et al. 1991), geotechnical properties

(Solheim et al. 1991), geophysical logs (Ollier and Mathis

1991), bio- and magnetic-stratigraphy (Baldauf and Barron

1991). Most of these works were aimed at deciphering the

onset of glacial continental climate and the initial devel-

opment of the ice sheet up to the early Neogene (Shipboard

Scientific Party 1989d).

ODP Site 1166 drilled during Leg 188 provided an

additional record on the shelf, illustrating in particular the

transition from East Antarctic preglacial to glacial condi-

tions (Shipboard Scientific Party 2001a,b). As for the

Cenozoic history of the shelf, a stratigraphic correlation

between Sites 742 and 1166 was proposed by Erohina et al.

(2004), supported by synthetic seismograms (Handwerger

et al. 2004). A comparison between Quaternary and Plio-

cene diatom biostratigraphy of Site 742 and 1166 was also

provided (Whitehead and Bohaty 2003). However, none of

these works considered in detail the information provided

by Site 739. Seismo-stratigraphic correlation of Sites 739,

1166 and 742 was considered only in a few general works

aimed at reconstructing the Plio-Quaternary history of the

Prydz Channel Fan (O’Brien et al. 2004) or depicting the

transition in the glacial history of the Prydz Bay (Cooper

and O’Brien 2004).

Recently, a review of the Cenozoic stratigraphy and

glacial history of the Lambert Graben and Prydz Bay

region has been synthesized from previous works (White-

head et al. 2006).

In this work we propose a detailed stratigraphic frame-

work resulting from data integration of Sites 739, 1166 and

742 (Fig. 9). It complements and refines the broad review

provided by Whitehead et al. (2006).

This framework recognizes 5 units within the topset

beds of seismic units PS.1 and the first units within PS.2. In

spite of the lateral variability shown in detail by the three

sites, the four upper units may be correlated along the

seismic profiles with the aid of log (P-wave velocity),

lithologic and age information.

UNIT A

The uppermost unit (upper few meters) consists of soft

Quaternary sediments (diatomaceous sand silt and dia-

micton) on top of the underlying firmer diamictites

(Fig. 9). This unit corresponds to lithologic unit IA in Site

1166 and I in Sites 742 and 739 (Shipboard Scientific Party

1989b, c, 2001a, b), and lies within seismic unit PS.1 of

Stagg (1985). It is not logged and not resolved in the

seismic profiles (Figs. 3, 4), though may be considered to

be represented by the low velocity layer within the seafloor

reflection (Figs. 7a, b).

UNIT B

The underlying unit, 60–110 m thick, consists of a homo-

geneous poorly sorted diamictite (Shipboard Scientific

Party 1989b, c, 2001a, b). It lies within seismic unit PS.1

and corresponds to lithologic unit IB in Site 1166 (Cores

1R to 12R, 3–106 mbsf), to unit II in Site 742 (Cores 2R to

14R, 5–115 mbsf), and to the upper part of the upper half

of unit II in Site 739 (Cores 4R to 10R, 24–80 mbsf). In

Site 739 this unit was not subdivided from the underlying

sediments because the poor core recovery in the upper

100 m (about 15%) prevented sample of the boundary. We

set the base of this unit at 80 mbsf, where logging data

recorded a change in lithology (Shipboard Scientific Party

1989b). This unit is characterized by relatively low P-wave

velocities around 2.2 km/s (Fig. 9) showing a certain

amount of variability within a downward trend that overall

is constant or slightly increasing. A fair degree of lateral

variability within this unit is suggested by the seismic data

showing discontinuous reflectors and channel-like features

(Figs. 3, 4). These features and the erosional truncations

are even more evident in the ‘‘depth-geometry’’ of the

migrated seismic profile (Fig. 8). Such lateral variability is

represented by the interfingering of low velocity layers

within the spatial distribution of P-wave velocity (Figs. 7a,

b). The age of this unit is late Pliocene to Quaternary in

both Sites 1166 and 742. It may well be the same in Site

739, where the stratigraphic placement is ‘‘tentative at

best’’ (Shipboard Scientific Party 1989b). The only three

valid samples there suggest a maximum age of late Plio-

cene (2R-CC), no older than early Pliocene (8R-CC) and

late Miocene or younger (10R-CC).

UNIT C

The underlying unit, 16(?)–30 m thick, is a stratified,

lithologically variable unit (Fig. 9). This internal variabil-

ity, combined with the moderate recovery (less than 50%

on average) resulted in appreciable differences in the three

sites. It lies within seismic unit PS.1 and corresponds to

lithologic unit IC in Site 1166, to unit III in Site 742, and to

the lower part of the upper half of unit II in Site 739

(Shipboard Scientific Party 1989b, c, 2001a, b). In partic-

ular: in Site 1166 the recovered part of unit IC is only 4 m

thick (Core 13R, 113–117 mbsf), but its top and base were

2000 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

not recovered. This unit may actually be up to 16 m thick

if the whole interval between the recovered parts of the

adjacent units is considered. It consists of two diatoma-

ceous clayey silt beds (about 15 and 60 cm thick) and

interbedded diamictites, dated to the upper Pliocene on the

basis of diatom biostratigraphy. In Site 742 four subunits

were ascribed to unit III, for a total thickness of 19 m

(within Cores 14R to 16R, 115–134 mbsf) (Shipboard

Scientific Party 1989c). Subunit IIIA is 12.5 m thick and

consists of weakly to moderately stratified diamictite.

Subunit IIIB is a 60 cm thick diatomaceous clayey silt bed

of late early Pliocene to early late Pliocene age. Subunit

IIIC is a 5.4 m thick weakly stratified diamictite similar to

subunit IIIA. Subunit IIID is a 70 cm thick boulder. In Site

739 this unit did not achieve recognition as a separate

subunit within unit II due to the poor recovery (about 5%).

It is 30 m thick (Cores 11R to 13R, 80–110 mbsf). As far

as it is known from the scarce samples and logging data,

this lithology consists of stratified diamictite with inter-

bedded sandy sediments with up to 25% diatoms of an

imprecisely determined Pliocene age. In all three sites this

unit is characterized on the logging data by a low velocity

layer with P-wave values lower than *1.9 km/s (Fig. 9).

This unit corresponds to the apparently continuous low

velocity layer at about 110 ms below the seafloor shown by

the spatial distribution of P-wave velocity (Figs. 7a, b).

The fair lateral continuity of this unit, locally interrupted

likely by erosion soon after deposition, would be also

suggested by the nearly continuous reflection that is clearly

seen in the seismic profiles (Figs. 3, 4). However, rather

than corresponding to a single layer, this strong complex

reflection may be caused by the composite effects of thin

and variable thickness interlayered soft and hard layers

(Erohina et al. 2004). It is also clear from the high reso-

lution profile Palmer 04 that the very continuous reflector

close to the base of PS.1 actually lies above the PS.1 base

sampled in Site 742 (Fig. 3). The reflector representing

this unit is marked as ‘‘C’’ in the depth profile of Fig. 8.

Evaluating the biostratigraphic evidence in the three sites,

we assign a generic late Pliocene age to this unit. In par-

ticular, the upper of the two diatomaceous beds in Site

1166 is dated to 1.8–2.0 to 2.1–2.5 Ma and correlates to

subunit IIIB of Site 742 (Whitehead and Bohaty 2003). The

lower diatomaceous bed in Site 1166, possibly separated

Fig. 9 The proposed reviewed Neogene stratigraphy along the continental shelf of the Prydz Bay region, obtained integrating P-wave velocity

logs, lithologic and biostratigraphic information from the ODP Sites 1166, 742 and 739

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 2001

123

from the upper one by a disconformity, is dated to 2.5–2.7

to 2.7–3.2 Ma, whereas older Pliocene strata appear to be

disconformably absent from Site 742. In Site 739 no

sample was available from Core 11R, whereas Core 12 R

contained a Quaternary diatom assemblage considered

downhole contamination (Shipboard Scientific Party

1989b). An early Pliocene diatom assemblage age was

initially suggested for Core 13R, which recovered 1.75 m

within the 106–115 mbsf interval. However, the early

Pliocene interval belonging to Neogene Southern Ocean

diatom (NSOD) zone 14–15 was clearly stated to occur

beneath the recovered part of Core 13R, and precisely at

111 mbsf (Shipboard Scientific Party 1989b; Baldauf and

Barron 1991). The age of this unit in Site 739 is hence

reasonably regarded as late Pliocene, in agreement with the

other sites.

UNIT D

The underlying unit, 12–38 m thick (Fig. 9), consists of

highly compacted diamictite (Shipboard Scientific Party

1989b, c, 2001a, b) that recorded preconsolitation stresses

of about 7,000 kPa (Solheim et al. 1991). It lies within

seismic unit PS.1 and corresponds to lithologic unit ID

(Cores 14R to 15R, 123–135 mbsf) in Site 1166, to unit IV

(Cores 16R to 20R, 134–172 mbsf) in Site 742 and to the

upper part of the lower half of unit II (Cores 14R to 17R,

110–140 mbsf) in Site 739. At Sites 1166 and 742 this is

the lowermost unit within PS.1, but not in Site 739. In Site

739 we set the base of this unit at 140 mbsf, in corre-

spondence of the erosional unconformity (load event 2)

determined on the base of consolidation tests (Solheim

et al. 1991). A small but distinct peak in the water content

was inferred to indicate a lithologic change in this interval,

an observation which was further supported by a change in

clast content and in undrained shear strength. In particular,

load event 2 was placed at the top of a thin layer of

apparently more fine-grained sediments that could repre-

sent the erosional remnants of a more distal (interstadial)

sediment (Solheim et al. 1991).This unit is characterized by

a high P-wave velocities, around 2.5 km/s (Fig. 9). This

high velocity unit is rather continuous along both dip and

strike profiles (Fig. 7a, b). However at Site 1166 it is much

thinner and eroded by the overlying unconformity

(Fig. 7b). The geometry of this unit in the depth profile is

represented by the concave reflection (red reflector) with

variable amplitude (Fig. 8). No biostratigraphic control

was provided by either Sites 1166 and 742. The age of this

unit is early Pliocene on the basis of Site 739 evidence. In

this site in fact, this unit lies within the early Pliocene

NSOD zones 14 to 15 (111–127 mbsf) and above early

Pliocene NSOD zone 12 from Core 18R downward

(Baldauf and Barron 1991).

UNIT E

The lowest unit, 34 m thick (Fig. 9), is present only in Site

739, where it lies just above the base of seismic unit PS.1,

in the lower half of lithologic unit II (Cores 18R to 24R,

140–174 mbsf).). It consists of very highly compacted

diamictite (Shipboard Scientific Party 1989b) that recorded

preconsolitation stresses of about 8,000 kPa and was

deposited in a more distal position with respect to the

overlying diamictite (Solheim et al. 1991). This unit is

characterized by a high P-wave velocities, around 2.5 km/s

(Fig. 9). It is gently SSE-dipping and terminates (by ero-

sional truncation) against the ridge around Shot 2150 on

line PB33–21, without reaching Site 742 (Figs. 7a, 8). Two

sub-units non resolvable on the seismic record may tenta-

tively be distinguished on the basis of the diatom contents:

the early Pliocene E1 with up to 25% diatoms in Cores 18R

to 19R belonging to NSOD zone 12 (Shipboard Scientific

Party 1989b; Baldauf and Barron 1991); the late Miocene

E2 with no or few diatoms in Cores 20R to 24R (Shipboard

Scientific Party 1989b; Baldauf and Barron 1991).

Older units

The underlying units lie within the prograding seismic unit

PS.2 and are essentially different from site to site. In

general these units are progressively older from Site 739, to

742, to 1166. Lower Oligocene glacial diamictite units III

to V were sampled at Site 739 (Barron et al. 1991). Glacial

diamictite units V and VI of Site 742 lie below the sedi-

ments sampled in Site 739, and are hence inferred to be

Eocene or earliest Oligocene in age (Barron et al. 1991).

Site 1166 drilled an older section than Site 742, and most

units sampled either do not extend between the two sites

(Erohina et al. 2004). In particular, Site 1166 sampled

glacial sediments within late Eocene to early Oligocene

glaciomarine proglacial unit II and late Eocene glacioflu-

vial deltaic unit III, whereas pre-glacial sediments were

sampled in late Cretaceous lagoonal unit IV and in Creta-

ceous low-relief alluvial plain unit V (Cooper and O’Brien

2004; Erohina et al. 2004).

Comparison with Lambert Graben and Prydz Bay

region–Cenozoic stratigraphy

The reviewed Neogene stratigraphy that we obtained by

integration of seismic, logging, lithologic and biostati-

graphic data of three shelf sites, essentially differs from

previous stratigraphies in the treatment of unit II of Site

739 and its correlation to Site 742 and hence Site 1166.

However, previous stratigraphic reconstructions never

attempted a direct correlation of these 3 crucial sites,

and were not assisted by an estimation of the spatial

2002 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

distribution of the logging properties and by a careful

geometric reconstruction of the lithologic units along

depth-migrated seismic profiles. Among the factors that

prevented a correct stratigraphy of Site 739 we recognize:

(1) the poor recovery in the upper 100 mbsf that precluded

the sampling and hence the definition of different facies.

These different facies were correctly identified within unit

II by logging information, but were unfortunately not

recognized by the seismic interpretation (Ollier and Mathis

1991); (2) initial errors in the age placement, even after

they were already corrected in the scientific results volume

(e.g. the age placement of loading event II of Solheim et al.

1991; see below); (3) some lack of update in the scientific

literature with regards to the evolution of the biostrati-

graphic zonations (e.g. the NSOD zone 12 was ascribed to

the late Miocene at the time of the scientific results volume,

but later assigned to the early Pliocene. However, sedi-

ments within that zone and originally place in the late

Miocene have been referred to that age until recently).

The detailed Neogene stratigraphy that we propose for

the Prydz Bay shelf essentially fits within more general

review of the Cenozoic stratigraphy of the Lambert Graben

and Prydz Bay region synthesized by Whitehead et al.

(2006). A comparison between the two stratigraphies sup-

ports, complements and refines the latter.

In our stratigraphy (Fig. 9) the units are named A to E

downsection. In that of Whitehead et al. (2006) 4 main

types of events are independently numbered in chrono-

logical order from the oldest. These main types of events

include: terrestrial deposits (T), marine deposits (M), ero-

sional unconformities (E), ice loading events (L).

The general consistency between the two stratigraphies

(Table 2) is suggested by the almost one-to-one match of

the units (and their relative boundaries) of our stratigraphy

to those of Whitehead et al. (2006).

The general Neogene glacial history for the Prydz Bay

shelf that is supported by both stratigraphies is the

following:

• Post-late Eocene—early Oligocene multiple glacial

advance;

• Miocene (to Early Pliocene) deposition of glaciomarine

sediments;

• Erosion of Prydz Bay Channel and initiation of the

Prydz Channel Fan;

• Late Pliocene to early Pleistocene multiple glacial

advances and deposition of glaciomarine sediments.

• The discrepancies, inherent in the incomplete and often

poorly dated records, lead to interesting considerations

and refinements.

• Post-Oligocene deposition occurred in Late Miocene

time (our unit E2). No evidence exists on the shelf for

Middle Miocene deposits. Miocene was sampled only

in Site 739. There, samples from Core 25R to 38R

downward were assigned an earliest Oligocene age, and

Cores 23R–24R were placed in the late Miocene,

though no samples were unfortunately available from

Core 24R (Shipboard Scientific Party 1989b; Baldauf

and Barron 1991; Barron et al. 1991). The first post-

Oligocene deposits on the shelf are hence the late

Miocene glaciomarine sediments (our unit E2), which

correspond to unit M6 of Whitehead et al. 2006

(Table 2). This fact does not necessarily discount a

possible Mid Miocene age for the widespread post-

Oligocene erosion surface (Base of seismic unit PS.1 in

Figs. 8, 9; surface E2 of Whitehead et al. 2006).

Table 2 Comparison between Neogene stratigraphy presented in this work and the one proposed by Whitehead et al. 2006

Neogene Prydz Bay shelf stratigraphy (this work) Cenozoic stratigraphy of the Lambert Graben and Prydz

Bay region (Whitehead et al. 2006)

Unit Proposed age Event Proposed age

A Quaternary M16–17 Holocene–late Pleistocene

Glacial erosion Late? Pleistocene E8 Late? Pleistocene

B Late Pliocene to Pleistocene L8 Early Pleistocene

Sharp, irregular, deformed contact in 742 Late Pliocene to early Pleistocene E7 Late Pliocene–early Pleistocene

C Late Pliocene (M9) M10–M11 Late Pliocene–early Pleistocene

Loading event III of Solheim et al. (1991), p. 12 Early late Pliocene L5 Early to late Pliocene

D Early Pliocene M8 Early Pliocene (NSOD 14-15)

Loading event II of Solheim et al. (1991) Early Pliocene E4, L4 (Late Miocene to) early Pliocene

E1 Early Pliocene (NSOD 12) M7 Early Pliocene (NSOD 12)

E1/E2 boundary Late Miocene E3 Late Miocene

E2 Late Miocene M6 Late Miocene

Base of PS.1 Post-early Oligocene E2 Post-late Eocene–Early Oligocene

Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007 2003

123

• The late Miocene glaciomarine sediments unit E2 (M6

of Whitehead et al. 2006) unconformably overlie the

post-Oligocene erosion surface (E2 of Whitehead

et al. 2006) and may be overlain by an erosion

surface. This erosion should correspond to that

produced by the late Miocene regional glacial

advances and identified as E3 by Whitehead et al.

(2006), which in this case would post-date the late

Miocene glaciomarine sediments in Prydz Bay

(Table 2) and likely also those in the Pagodroma

Group (Whitehead et al. 2003, 2004). This is possible

since the age ranges of the sediments (M6) and of the

erosion surface (E3) have been estimated to be very

similar, between 10.7–8.5 Ma and 10.7–9 Ma, respec-

tively (Whitehead et al. 2006).

• The late Miocene erosion (our E1/E2 boundary; E3

surface of Whitehead et al. 2006) is overlain by a

glaciomarine unit (our E1) dated to the early Pliocene

(NSOD zone 12), which in turn is overlain by an

evident erosion surface (our D/E1 boundary, Figs. 8,

9). This surface in Sites 742 and 1166 coincides with

the base of seismic sequence PS.1 (Stagg 1985),

since it eroded the early Pliocene-Miocene record of

glacial advances and deposition on the shelf. D/E1

boundary was produced by the glacial advance

accompanied by sediment compaction identified as

load event II by Solheim et al. (1991). It should

hence correspond to surface E4 of Whitehead et al.

(2006), which in this case would post-date the early

Pliocene sediments placed within NSOD zone 12

(Table 2). Load event II, placed by Solheim et al.

(1991) at 140 mbsf in Core 17R of Site 739, was

attributed to the late Miocene because the samples of

Core 17R were initially assigned (only tentatively) to

this age on the base of the occurrence of D. Hustedii

(Shipboard Scientific Party 1989b). The assignment

of load event II to the late Miocene survived to the

present day notwithstanding the underlying Cores 18

to 19R have been undoubtedly placed to the (early

Pliocene) NSOD zone 12 since 15 years (Baldauf and

Barron 1991). Our placement of load event II within

the early Pliocene does not contrast with the other

constrains reviewed by Whitehead et al. (2006) and

makes some events younger, limiting, for example,

the age of E4 (previously ascribed to span between

4.1 and 6.3 Ma) to span between 4.1 Ma (the

minimum revised age of the overlying Sørsal For-

mation; Whitehead et al. 2004) and 4.9 Ma (the

maximum age of the underlying sediments in Site

734).

• Erosion surface D/E1 is overlain by early Pliocene

sediments belonging to NSOD zones 14 to 15, which

in turn are overlain by erosion surface C/D (Figs. 8,

9), in turn overlain by the late Pliocene unit C.

Surface C/D, at 110 mbsf in Site 739, is dated to late

early/early late Pliocene, and corresponds to loading

event III of Solheim et al. (1991) and hence to

loading event L5 (Table 1) of Whitehead et al.

(2006). It also corresponds to surface PP-12 of

O’Brien et al. (2004) and to reflector A of Mizukoshi

et al. (1986), which was tied to Site 739 between 106

and 130 mbsf by O’Brien et al. (1995). This event is

one of the most significant in the Neogene glacial

history of the region. It corresponds to the cutting of

Prydz Bay Channel and initiation of the Prydz

Channel Fan (Cooper et al. 1991b; Cooper and

O’Brien 2004). The initiation of the Prydz Channel

Fan is marked in the seismic units of the continental

shelf and slope by a significant change in the

geometry, towards rapid progradation of the shelf

edge along the channel axis and downlap at the base

of the slope (O’Brien et al. 2004). This geometry

changes are observed also in other Antarctic margins

at presumably similar ages (Rebesco et al. 2006).

These authors have analyzed six different areas along

the Antarctic margin: Antarctic Peninsula, Prydz Bay,

Weddell Sea, Wilkes Land and Eastern and Western

Ross Sea. These sectors of the continental margin

share the same characters: (1) enhanced progradation

in the form of steep sedimentary wedges building

above pronounced truncation surfaces; (2) prominent

downlapping of these wedges at the base of the

continental slope; and (3) significant decrease in

sedimentation rates on the continental rise. The

common driving force has been inferred to be the

Late Neogene transition of ice sheet regime to

modern polar conditions. Previous placement of this

event (reflector A, PP-12, initiation of Prydz Channel

Fan and Paleo shelf-break 4) to the late Miocene (e.g.

Cooper et al. 2001) was based on an erroneous

stratigraphy of Site 739. However, the reviewed

stratigraphy of Site 739 that we obtained by integra-

tion of seismic, logging, lithologic and biostatigraphic

data of three shelf sites, fits very well within the

broad Cenozoic stratigraphy of Whitehead et al.

(2006). Conversely, the coincidence that we identify

between PP-12 and Load event III (Solheim et al.

1991), and our revised age of these events provide an

answer to the uncertainty for the age of the initiation

of the Prydz Channel and Prydz Channel Fan. This

event in fact was attributed to late Miocene or to

early Pliocene in the review of Whitehead et al.

(2006). We constrain surface PP-12 (our C/D bound-

ary) to the Pliocene, between 4.1 and 2.5 Ma, as

suggested for loading event L5 of Whitehead et al.

(2006).

2004 Int J Earth Sci (Geol Rundsch) (2009) 98:1991–2007

123

Conclusions

To revise the Neogene stratigraphy of the Prydz Bay

continental shelf we:

• reprocessed and analyzed a dataset constituted by 3 low

resolution multichannel seismic reflection profiles and

1 single-channel profile;

• reviewed the existing logging, lithologic, physical

properties and biostratigraphic data of three sites

(ODP Leg 188 Site 1166 and ODP Leg 119 Sites 739

and 742);

• estimated (using the Emerge software) the spatial

distribution (along the crossing seismic profiles) of

the P-wave velocity measured in the three sites;

• used the resulting velocity distribution in the depth-

migration of the dip-oriented seismic profile (PB33-21)

to obtain a reliable geometry reconstruction;

• integrated the information derived from the above steps

(seismic profiles, well data, velocity distribution, geo-

metric reconstruction) to assist the correlation among

the three sites of the stratigraphic information;

• compared the resulting detailed Neogene stratigraphy

of Prydz Bay continental shelf with the recently

reviewed broad Cenozoic stratigraphy of the Lambert

Graben and Prydz Bay region by Whitehead et al.

(2006).

As a general result, the Neogene stratigraphy proposed

by Whitehead et al. (2006) is confirmed (with minor

internal re-adjustments), but we find some events are

younger by up to a few Myrs, assigning a more precisely

age to the following events:

• the first shelf deposits above the post-early Oligocene–

late Eocene glacial advance are dated to late Miocene

instead of mid Miocene;

• the glacial advance that produced the load event II

(Solheim et al. 1991) and that eroded the post-early

Oligocene–late Eocene sediments in Sites 742 and 1166

is placed in early Pliocene time (between NSOD zones

14 to 15 and 12) rather than at the transition between

late Miocene and early Pliocene;

• the erosion of Prydz Bay Channel and the initiation of

Prydz Channel Fan (marked by reflector A of

Mizukoshi et al. (1986) and PP-12 of O’Brien et al.

2004) is dated to early late to late early Pliocene,

instead of at the transition between late Miocene and

early Pliocene. The age of this event in Prydz Bay is

consistent with the age suggested by Rebesco et al.

(2006) for similar events recognized in different

Antarctic margins, and inferred to result from the

transition to modern, ‘‘dry and cold-based’’, polar ice

sheets.

Acknowledgments Research was conducted within the project

‘‘Analysis of the physical properties relevant to the seismic stratig-

raphy, ODP Leg 188–Leg 119, Prydz Bay funded by the Italian

Programma Nazionale di Ricerche in Antartide (PNRA). The logging

data used in this work are furnished by the Ocean Drilling Program

(ODP). We thank German Leitchenkov for his contribution in pro-

viding Russian seismic data collected by within the 32nd and 33rd

Soviet Antarctic Research Expedition.

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