Marine Geology 141(1997) 91-109
Sedimentary processes across the continental rise of the southern Antarctic Peninsula John P. McGinnis a, Dennis E. Hayes b,*, Neal W. Driscoll ’ a Amerada Hess Corporation, Houston, TX 77002, USA b Lamont-Doherty Earth Observatory, Columbia University, Department of Earth Sciences, Palisades, NY 10964, USA ’ Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 14 May 1996; accepted 15 April 1997
Abstract
A series of large sediment mounds have been identified along the Pacific portion of the Antarctic Peninsula continental rise. These mounds are composed of sediment delivered to the continental rise during the advance and retreat of grounded ice across the shelf. The stratigraphic development of one of these sediment deposits, the Tula sediment mound, is examined to investigate how the onset of glaciation influenced the deep-sea depositional environment along this portion of the margin. The strata1 relationships, associated facies distribution, and the physiography observed along the southern Antarctic margin reflect the waxing and waning of the Antarctic ice sheets; various processes erode, transport, and deposit sediment along the outer shelf, slope, and rise throughout a glacial cycle. A deep-sea erosional unconformity is apparent at the base of the Tula deposit. This surface may reflect the first onset of intensified bottom water circulation along the margin perhaps induced by the tectonic opening of the Drake Passage. The Tula sediment mound is comprised predominantly of canyon/overbank systems. Evidence for the onset of canyon cutting and the development of the thick overbank deposits (> 2 km) is found immediately above the deepsea erosional unconformity. Relating increased canyon cutting across the continental rise to the fluctuation of ice across the shelf implies that the onset of predominantly glacial conditions commenced soon after the onset of intensified bottom water circulation along this margin. The volume of sediment associated with the Tula deposit and the apparent ‘point source’ distribution of the channel systems suggests that much sediment was transported to the slope and rise perhaps by meltwater processes through these canyons. 0 1997 Elsevier Science B.V. Keywords: Antarctic Peninsula; continental sediment mounds
rise; glacial marine sediments; paleoceanography;
1. Introduction
Investigators have previously described the strong contribution of downslope processes in the construction of the Antarctic continental slope, * Corresponding author. Tel.: + I (914) 3658470; fax: + 1 (914) 3658156; e-mail:
[email protected] 0025-3227/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO25-3227(97)00056-X
margin sedimentation;
rise, and abyssal plains (e.g., Drewry and Cooper, 1981; Wright and Anderson, 1982; Wright et al., 1983). Glacial-marine processes are regarded as the dominant process-regulating canyon formation across the shelf, slope, and rise and the variation of sedimentation rates to the basin. These processes, which have been interpreted predominantly as mass gravity and debris flows, must effectively
92
.I P. h4cGinni.c CTal.
! Mark
transport large volumes of sediment from the continental shelf and slope to the deep basin. Fineto coarse-grained sands have been documented at abyssal depths, hundreds of kilometers from the continental slope (e.g., Wright and Anderson. 1982; Baegi, 1985). and large channel-levee systems are extant across the continental rise and abyssal plain in many regions as is typical of lowlatitude margins. The strata1 relationships, associated facies distribution, and the physiography observed along the Antarctic margin predominantly reflect the fluctuation of the Antarctic ice sheets and the varied processes that erode, transport, and deposit sediment along the outer shelf and slope throughout glacial-interglacial cycles. For example, during a glacial maximum, sediments are scraped and bulldozed by the glacier and are disgorged at the edge of the grounded glacier in close proximity to the shelf edge. As a result, large obliquely prograding wedges of glacially eroded material occur along the outer shelf and slope downlapping onto the pre-existing deposits. Conversely, during periods of ablation, glaciofluvial sediment transport processes become more prevalent across the margin as evidenced by channels and thick channel overbank deposits. The planform of these channel overbank deposits indicate that they formed by downslope gravity flows. In addition, the thickness and distribution of these deposits along the southern Antarctic Peninsula requires the existence of large drainage networks, perhaps related to meltwater processes. The differential thickness of the overbank deposits away from the channel implies that these channels/canyons were not formed by slope failure and headward erosion alone. Taken together, these observations suggest that the spatial and temporal stratigraphic development of the Antarctic margin can best be explained in terms of how sediment was supplied to the evolving margin throughout a glacial/interglacial cycle. Mechanical erosion and oblique progradation dominate during glacial advances; during the end of glaciation and early deglaciation glaciofluvial networks associated with the ablating ice and possibly climatic variations becomes the dominant mechanism supplying sediment to this margin. Calving of icebergs and the development of drop-
Grolog~~ 141 (1997) YI-109
stones accompanies the early ablation of the glaciers, but in our model it is not the primary sedimentation mechanism associated with glacial retreat.
2. Southern Antarctic Peninsula tectonics and geology
During early 1991, the Lamont-Doherty Earth Observatory and the University of Texas Institute for Geophysics acquired approximately 6100 km of multi-channel seismic reflection, gravity, magnetic, and swath bathymetry data along the Antarctic Peninsula. A portion of the survey, conducted aboard R/V A4uurice Ewing, concentrated along the Pacific margin of the Antarctic Peninsula (Fig. I), and consisted of 11 regional reconnaissance seismic reflection profiles totaling greater than 2500 km. These data were acquired using a 60-channel, 3.1 km streamer, and a tunedairgun array with a total source capacity of N 133L; 12-s sections were recorded at a 4 ms sampling interval. The seismic reflection sections used in this study are 60-fold, common midpoint stacks. Basement topography along the southern Antarctic Peninsula continental rise is typically irregular. The major fracture zones identified from seismic reflection sections and offset magnetic lineations include from north to south, the Adelaide, the Tula, the Alexander, and the Heezen (Fig. 1). Marine magnetic anomalies along the Antarctic Peninsula show that the oceanic crust increases in age away from the margin, which indicates that the original Phoenix plate has been subducted beneath the peninsula (Barker, 1982). Relative motion between the Phoenix and Antarctic plates was such that the fracture zones were subducted nearly perpendicular to the peninsula. Subsequently, collision of otIsset ridge segments with the continental margin led to the progressive northward cessation of subduction along the Bellingshausen margin of the peninsula (Barker, 1982). Subduction initially terminated in the southern portion of the peninsula during the Eocene (- 50 Ma), and due to several large leftlateral offsets of the ridge axis, occurred then in a
J.P. McGinnis er al. / Marine Geology 141 (1997) 91-109
-100'
93
-60’ -76’ 1
-74’ I
-72’ I
-68”
Fig. 1. Bathymetry, fracture zones, and track location map of the southern Antarctic Peninsula. Small circles represent piston and gravity cores collected by the Elranin across the outer continental shelf, slope, and rise. The bold solid lines and corresponding numbers indicate figures illustrating seismic reflection profiles and interpretations. A&Z, AxSM= Alexander fracture zone and sediment mound, respectively; 77’2, TSM=Tula fracture zone and sediment mound; AdFZ, AdSM=Adelaide fracture zone and sediment mound. Insets: DSDP site locations; other geophysical track lines in the region used to generate the bathymetry map.
time-transgressive fashion to the northeast (Barker, 1982). Active, albeit slow, subduction may still be continuing beneath the Shetland Islands (Pelayo and Wiens, 1989). The age of oceanic crust in the region of study, based on identified magnetic anomaly lineations, ranges from Late Eocene to Early Oligocene south of the Tula fracture zone, middle Oligocene to Early Miocene between the Tula and Adelaide, and Early to Mid-Miocene just to the north of the Adelaide fracture zone (Herron and Tucholke, 1976; Barker, 1982; Larter and Barker, 1991; Hayes, 1991). The
Tula fracture zone is located -40-50 km to the north of the Tula deposit (Fig. 1). The geomorphology of the continental shelf, slope and rise along the Antarctic Peninsula is typical of the Antarctic continental margins elsewhere. Bathymetric profiles across the middle to outer continental shelf indicate that the shelf shoals in a seaward direction. Water depths often remain deeper than 400 m, and interval velocities from sonobuoys deployed across the shelf indicate that the velocities of shallow sediment are >2 km/s. These relatively high velocities suggest that the
94
J. P. McGinnis et al. : Murinr Geology 141 (1997) 91-109
sediments are overcompacted which may be due to the previous stages of grounded ice. Large troughs, which formed during the advance of grounded ice (Bart and Anderson, 1995), occur across the continental shelf (Fig. 2). Throughout the study area, the continental slope dips steeply, ranging between 10” and IS”, but decreases to -5-6” farther to the south, west of Alexander Island. The slope-to-upper rise transition occurs at water depths of -2500-3000 m. Three large sediment mounds, here termed the Alexander mound, Tula mound, and Adelaide mound, have been identified across the lower slope and rise (Fig. 1; McGinnis and Hayes, 1995). The Tula deposit is approximately 100 km wide, and extends - 100-l 50 km across the continental rise. This sediment deposit rises to -2700 mbsl and is asymmetric in cross-section with its steepest side (-4-5”) facing to the southwest. From the crest of the southern face, the mound dips slightly to the north where it again attains slopes of before deepening at - l-2. m mto the nortii:i canyon system. Active canyons do not appear to cut across this feature. Sediments deposited near the top of the Tula mound are characterized seismically by acoustically laminated reflectors, with no current-controlled bedforms recorded in the seismic reflection and 3.5 kHz data. Strong diffractive seismic returns are indicated near the seafloor surface on the 3.5 kHz records across the steep faces of the Tula deposit. These diffractions are absent across the top of the sediment mound, and the good penetration shown on the 3.5 kHz records suggests that fine-grained sediments ( - 0.06-0.07 twtt, - 50 m) are likely present there. The seismic reflection data indicate a complex history of canyon cutting across the upper continental rise, and thick overbank deposits related to these canyons have contributed to the sediment accumulation patterns along this margin (Fig. 3; see McGinnis and Hayes, 1995). Total sediment thicknesses along the rise range from -2.0 to 4.5 km (- 1.5-3.0 s twtt). The Tula deposit is flanked on both sides by large canyon systems which extend seaward across the lower rise (Fig. 1). The southern canyon has not been active long, as indicated by the limited extent of the overbank deposits observed on both sides of this
canyon system. The southern overbank deposit is interlayered with the younger facies units across the northern portion of the Alexander deposit and it appears that the canyon may have been active only during the most recent phase of evolution of the Alexander and Tula deposits. The northern canyon system, which separates the Tula and Adelaide sediment mounds, is more complex and has been active for a longer period of time. Well developed overbank deposits are not observed in association with this canyon system across the upper rise. A channel-levee system, most likely related to this canyon, is identified from R/V Conrad and R/V Eltanin seismic records across the lower continental rise (Tucholke and Houtz, 1976). Folding and faulting of strata indicate that slumping into the canyon system has been recently active.
3. Sediment mound formation Across the upper continental rise, downslope processes in the form of canyon/overbank formation, control the stratigraphic development of the Tula sediment mound, and overbank deposits comprise the bulk of the Tula deposit. Along-slope processes have also played a role in the formation of this deposit. Current-controlled depositional structures are described from seismic reflection records of the Tula mound farther out on the continental rise (Rebesco et al., 1994), and may have influenced deposition during the early history of mound formation (McGinnis and Hayes, 1995). Bottom current influence, however, plays only a minor role during the formation of the large sediment mounds along this margin. This transition in sedimentation from downslope processes to along-slope processes is most likely a function of proximity to sediment sources and sedimentation rates. The base of the Tula sediment mound is defined seismically by a regional deep-sea unconformity (ETU, early-Tertiary unconformity; Figs. 3 and 4). This unconformity, characterized by local truncation of seismic reflectors beneath this surface, indicates an increase in erosional intensity to the south beneath the Alexander deposit (McGinnis and Hayes, 1995). The forma-
J.P. McGinnis et al. / Marine Geology 141 (1997) 91-109
95
I , , , , I
.
\
\
*. .
SAP-1 WO
Y
l. .
...
400 8zaJ8200
81!30
am
8888 mm
1950
7mo
distance (km) Fig. 2. Bathymetry map highlighting: (a) shelf relief including SClLSC4 shelf troughs along transects SAP-9 and SAP-IO; and (b) lithologic log of DSDP site 3.25drilled to a total depth of 718 m on the lower continental rise (Hollister et al., 1976). Middle Miocene hiatus or interval of very slow sedimentation is indicated. Ice-rafted pebbles first occur just prior to the Miocene deep-sea hiatus. Arrows on map indicate large troughs which extend across the shelf (see Bart and Anderson, 1995). SC4 corresponds to the Marguerite Trough of Bart and Anderson (1995).
tion of current-controlled structures directly overlying the ETU within the Alexander deposit indicates that this unconformity may be a product
of intensified bottom currents along this portion of the southern Antarctic Peninsula continental rise (Tucholke, 1977; McGinnis and Hayes, 1995).
CDP
I
400 r---7_‘m~T-T_
SAP-7
profile (SAP-T)
across
2800 , ids_
and interpretation
Slump 2000 ,.__1600 ETCH__ _r - 2400) SGZlfpS Slumu scams
Fig. 3. Seismic reflection
800
3600
the Tula sediment
3200 --T--T~~i~-i-T~
mound.
-
4000
See Fig.
Slumo
4800
1 for location
block
4400
5200
_
_ ._ ~~ -
I\_--
__---
9400
~. -
.-_
~- --
9000
- -
?-L
-high
-
7800
.__..~
Oceanic basement
6200 1
-
ye--~
7400
7000
6600 I
NE
across the Tula sediment mound which extends across the profile shown. See Fig. 1 for location.
- ~ - -
\
6600
Fig. 4. Seismic reflection profile (SAP-3) and interpretation
7,0 :
-c-r_ _ ~~ _
CDP 9600
I
SAP-3
98
J.P. McGinnis rt al. : Mu&e
During formation of the ETU, sedimentation rates were low and along-slope processes controlled the depositional environment along much of the continental rise. The apparent scarcity of canyons across the upper rise prior to the formation of the ETU, and the extensive lateral continuity of reflectors within the older seismic units, suggests that sedimentation into the outer basin during this time was primarily the result of lower-energy hemipelagic deposition. Any terrigenous input was probably limited to that related to turbidites sourced from the upper slope and outer shelf. The ETU marks an important seismic transition separating a lower seismic unit characterized by parallel to subparallel, low-amplitude reflectors from an upper unit characterized by increased reflector variability, ranging from relatively highamplitude, acoustically laminated reflectors to very chaotic, discontinuous reflectors. A series of large, relict canyons has been identified within the Tula sediment mound above the ETU surface (C 1-C4, Figs. 3 and 4). In this study, the identification of canyons is based on the following strata1 characteristics. (1) The formation of an erosional trough with an irregular basal boundary, filled with material whose basal high-amplitude reflectors likely represent initially deposited coarse-grained sediments. These high-amplitude reflectors are subsequently overlain by a more transparent, often chaotic seismic facies which appears to onlap the canyon walls. (2) The development of overbank deposits which display acoustically laminated to sub-parallel reflectors and which thin by downlap away from the canyon. These facies units are terminated by erosional truncation into the canyon. The earliest canyons (Cl and C2) identified seismically within the Tula mound, occur immediately above the ETU unconformity (Figs. 3 and 4). These two canyons are believed to represent the same system based on their spatial relationship with respect to the ETU unconformity, and the similar southwest lateral displacement of Cl to C2 on both sections. Overbank deposits associated with these canyons are best developed along the southwest sides of the channel margins. This style of overbank formation is consistent with Coriolis forcing of sediment-laden flows to the left in the Southern Hemisphere, as described by Tucholke and Houtz (1976). Initial migration of
Geology 141 ( lYY7) Yl-IOY
the canyon to the southwest indicates that turbidity currents passing through these early canyons were predominantly erosional in nature, cutting into the base of the southwestern channel banks. These deposits are up to - 1000 m thick, and thin by downlap to the southwest over a distance of 40 km. The internal strata1 geometries reveal sub-parallel reflectors which exhibit only minor variability across the width of the deposit. The crests of the overbank deposits are located -20 to 25 km to the southwest of C2 canyon. Farther to the west, the overbank deposits thin and merge with other sediments, perhaps turbidites and hemipelagic sediments, which have partially infilled the bathymetric depression between the two mounds. Reflectors within this unit lap onto the overbank deposits, indicating that the Tula deposit was structurally high during this phase of formation. The unconformity separating the canyon from its overbank deposits is distinctive on the seismic sections. This unconformity, readily identified on each of the strike lines, is characterized by erosional truncation of the underlying reflectors and onlap of the subsequent ‘channel fill deposits’ onto this surface. The apparent curvature observed associated with this unconformity surface is largely caused by velocity pull-up due to shoaling of the overlying topography. The dip associated with this unconformity ranges between 2.0” and 3.5”. Along the northeast sides of the channel margins, the overbank deposits are thin and poorly developed. The youngest canyon system identified on seismic profile SAP-7, C4, is well imaged (Fig. 3). Both the northeast and southwest levees are preserved, and are characterized by low-amplitude reflectors which thin by downlap away from the canyon. These overbank deposits are - 500 m at their thickest point and extend for - 30-40 km in both directions. Initial deposition into the C4 canyon is reflected by a strong-amplitude, chaotic seismic facies which onlaps the canyon in both directions. The seismic unit deposited above the C4 canyon system is characterized by acoustically laminated, parallel to sub-parallel reflectors. This unit exhibits only minor thickness variations parallel to the margin (Figs. 3 and 4), and a seismic reflection profile through the center of the Tula mound suggests that this unit is an amalgamation of
J.P. McGinnis et al. /Marine
overbank deposits associated with canyon development across the upper rise (Fig. 5). Large canyons (C4-C6) exist within this upper unit across the upper rise, and for each phase of canyon cutting, these systems progressively stepped towards the continental slope. Canyon C4 is located at CDP 700, canyon C5 is located -25 km towards the slope at CDP 1800, and canyon C6 is - 15 km from the base of the lower slope. The overbank deposits associated with each of these canyons progressively filled the previous canyon (Fig. 5). Comparing this landward migration of canyons with the present-day bathymetry suggests that these features were progressively deflected to the northeast. The bathymetric data shows a topographic low that extends sub-parallel to the slope for - 50 km before turning toward the deep basin. The progressive deflection of canyons to the right is similar to that described by Tucholke and Houtz (1976) and Anderson et al. (1986) for canyon cutting across the continental rise farther to the west along the Antarctic Peninsula and within the Weddell Sea, respectively. Tucholke and Houtz (1976) relate eastward channel migration here to preferential deposition of sediments on the western levees as a result of Coriolis forcing. Preferential deposition across the western banks results in the building of these deposits to the point that they ultimately diverted the channel axis to the right. The thicknesses of the C6 overbank deposits are approximately twice the overbank thicknesses of the previous two canyons, C4 and C5. This increase suggests either an increase in sediment supply during the formation of the C6 system, or that the present-day locations of the canyons on the lower slope are such that continued downslope activity is subsequently re-established within the same canyon system. Minor amounts of erosion are observed within the C4 and C5 overbank deposits; however, the erosion is not sufhcient to explain the observed thickness variations between C4 and C5 and the subsequent C6 overbank deposits.
4. Relating sediment mound formation to the glacial environment
In the lower latitudes, the supply of terrigenous sediment to canyons is typically modulated by
Geology 141 (1997) 91-109
99
eustatic processes (e.g., Normak, 1978; Shanmugam and Moiola, 1982). This mechanism, however, is difficult to invoke for the Antarctic margins considering the shelves are so deep (>400 m), and therefore not exposed by Tertiary eustatic changes. In addition, these shelf depths also preclude any significant reworking of shelf sediments by wind-generated currents and storms. An alternative process must therefore be called upon that can regulate the supply of sediment to canyons across the slope. The formation of thick canyon-related overbank deposits across the continental rise, in addition to the presence of sands on the abyssal plain along the banks of recent submarine canyons (e.g., Wright et al., 1983; Baegi, 1985), demonstrates that large volumes of sediment were transported downslope across the southem Antarctic Peninsula rise through these canyon systems. These canyon/overbank systems reflect ‘point source’ sediment contributions (Bart and Anderson, 1995), rather than the ‘line source’ sedimentation patterns previously ascribed to bulldozing by oscillating grounded ice sheets (e.g., Haugland et al., 1985; Bartek et al., 1991; Latter and Cunningham, 1993). At least five distinct canyon systems can be identified on the seismic reflection data that have crossed the Tula deposit. Depositional processes governing sediment supply to the basin must therefore, be episodic and capable of supplying large volumes of fine- to coarsegrained sediments to the rise and abyssal plain. To account for the huge lateral extent of the Weddell Fan, which contains a significant portion of sandy turbidites, Wright and Anderson (1982) suggested two alternative processes controlling the supply of sediment to canyon heads across the upper continental slope. The first process attributes the supply of sediment to the canyons by a combination of sediment gravity flow processes and by strong contour currents along the outer shelf and upper slope. Wright and Anderson (1982) argue that slumps are common on the upper slope and that as these slumps move downslope they experience shear that results in debris flow transport. Continued shearing of the debris flow and incorporation of water results in the transition to turbidity currents that transport sand, silt, and clay to the base of the
I I
I
I
I
,I
I
I
I
I
I
I
I
I
I
I
‘I I
I I
;I :
I
I I
‘I !
I
I I
I
I
I
I
I
I
I
I
I
.I
I
I
I
I
I
I
I
I I
1; II
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
(W)
Spuooas
J.P. McGinnis et al. J Marine Geology 141 (1997) 91-109
slope and beyond. Bottom currents, often associated with Circumpolar Deep Water (CPDW), can exist at sufficiently high threshold velocities to transport sands along the Antarctic shelf and upper slope in the Ross Sea (Jacobs et al., 1970; Dunbar et al., 1985) and sediment cores from the Pennell Coast, George V Coast, and northwest Weddell Sea all penetrated sands and residual glacial marine sediments which record the influence of strong bottom currents along the outer continental shelves and slopes (Anderson and Smith, 1989). These currents erode, laterally transport, and redeposit sediment along the shelf into the canyon systems, and/or transport sediment downslope to the canyons which extend obliquely along the base of the continental slope and rise. Anderson and Smith (1989) cite one example on the continental shelf offshore of Pennell Coast where volcanic sands of known origin have been transported approximately 60 km along the outer shelf by contour currents. The second mechanism proposed by Wright and Anderson (1982) involves the transport of sediment to the slope by subglacial meltwater streams flowing beneath the Antarctic ice sheet during glacial maxima. The occurrence of turbidite sands, which occur at or near the seafloor and display glacial surface-textures, in the Weddell basin prompted Wright and Anderson ( 1982) to suggest that these sediments were deposited long after the ice sheet formed. They proposed that once the ice sheet reached the edge of the continental shelf, subglacial meltwater streams could flow directly to the continental slope, effectively draining large subglacial regions and delivering large volumes of sediment to the continental rise and abyssal plain. Seismic reflection data and cores taken across the continental shelf (Kennedy and Anderson, 1989; Pope and Anderson, 1992; Bart and Anderson, 1995), rise and abyssal plain (Goodell, 1968; Hollister et al., 1976; Baegi, 1985), allow the geologic and constructional history of the Tula deposit to be inferred. The downslope trend of the channel overbank deposits indicate that they formed by gravity flows. In addition, the thickness and distribution of the sediment mounds along the southern Antarctic Peninsula requires the existence of large drainage networks, most likely involving
101
meltwater processes. The differential thicknesses of the overbank deposits away from their channel sources implies that these channels/canyons cannot be formed by slope failure and headward erosion alone. Consider first, a single relative sea-level cycle tied directly to the advance and retreat of an ice sheet across the continental shelf. The major changes in depositional processes and the geologic implications for these changes during this cycle are inferred and depicted in Fig. 6. Consider a time of high relative sea-level (Fig. 6, time a), when the ice is well inboard of the continental shelf edge. Sediment is deposited across the inner shelf in the form of gravity flows and hemipelagic rain, partially filling the previously formed deep glacially carved troughs (e.g., as identified across Marguerite Bay; Kennedy and Anderson, 1989; Pope and Anderson, 1992). The inner portion of the shelves are deep (>400 m), and the landward regional topographic gradient, characteristic of many Antarctic shelves, serves to restrict sedimentation to the inner shelf regions along the coastline (see Bart and Anderson, 1995). Terrigenous sediment supply to the basin seaward of the slope is relatively low during this time, as much of the sediment available for deposition is sequestered on the inner shelf. Deposition across the continental rise therefore involves predominantly hemipelagic sediments and turbidites sourced from the upper slope and outer continental shelf. Since total sediment input during glacial retreat is low, the effects of bottom currents in eroding and redistributing material along the rise are relatively unmasked; consequently the continental rise is expected to be characterized by hemipelagic deposition or by deep-sea erosion where bottom currents are active. As the ice begins to advance across the shelf, relative sea-level begins to fall (Fig. 6, time b). Sedimentation is now concentrated in front of the advancing ice sheet, being subsequently eroded and reworked seaward as the grounded ice continues to migrate towards the shelf edge. Glacialmarine muds deposited during the previous glacial retreat are eroded from the shelf and incorporated into the sediment package at the front of the advancing ice sheet. The existence of glacial scour-
J. P. McGinnis et ul. / Marine Geology 141 (1997) YI--109
102
Mass
Depositional Process
Debris Overbank
Flows
Formation
Depositional Environment high
retreat
t
t
Relative
Ice Sheet
sea-level
I
advance
high
Downslope Contribution to Continental Rise
t Sediment supply
I low
Sediment Sediment Trapped Trapped On Beneath Twwd On Shelf Shelf Ice Shelf Sediment
Fig. 6. Conceptual diagram depicting relationship of depositional processes, environments, and products: (a) relative sea-level change is a consequence of the advance and retreat of the ice shelf across the continental shelf; (b) changing downslope contribution of sediment to the slope and rise is due to the advance of ice to the shelf edge and initial retreat of ice. Increased current-controlled erosion is predicted during periods of low sedimentation to the basin.
ing and sediment erosion is supported by numerous glacial unconformities on the adjacent continental shelf (Bart and Anderson, 1995). While sediments remain restricted to the shelf, sediment accumulation rates would remain low across the continental slope and rise. As relative sea-level continues to fall (Fig. 6, time c), and the ice approaches the shelf edge, deep-sea sedimentation rates increase dramatically across the continental slope and upper rise as slumps and debris flows are initiated (Fig. 7a). Sedimentation across the lower slope and upper rise during this period should reflect high-energy, poorly sorted, matrix-supported debris flows comprised chiefly of basal tills and diamicts. These units are typically characterized on seismic reflection profiles by progradational topsets (where preserved) across the outer shelf, and as highamplitude, steeply dipping, foresets across the
slope. These prograding foreset beds rapidly thin downslope and terminate by downlap onto a preexisting sequence boundary across the lower slope and rise (DLS, Figs. 5 and 7; e.g., Larter and Cunningham, 1993). The apparent restriction of these units to the lower slope and upper rise suggests that sedimentation farther out in the basin may remain low. Based on seismic reflection data, it has been argued that farther to the north along the Antarctic Peninsula, these progradational units are deposited from a ‘line source’ reflecting the advance of the ice sheet toward the outermost shelf along the margin (Larter and Cunningham, 1993). As the ice sheet advances to and across the shelf break during the glacial maximum (Fig. 6, time d), sedimentation rates across the lower rise and abyssal plain are low as much of the shelf is entirely covered by ice and the only material
J. I? McGinnis et al. / Marine Geology 141 (1997) 91- LO9
-I-
103
Akttwater
Fig. 7. Conceptual diagram depicting depositional processes predominant (a) during maximum glacial advance to the outer continental shelf edge resulting in the deposition of large obliquely prograding wedges of glacially eroded material along the outer shelf and slope downlapping onto the pre-existing sequence boundary; and (b) during periods of ablation, when fluvial processes become prevalent in transpotiing sediments to the continental rise; extensive ‘point source’ channeling and thick channel overbank deposits result. Figure modified from Uchupi and Swift (1991).
available for deposition is the sediment entrained at the base of the ice. At DSDP Site 325 located on the lower rise, the first appearance of ice-rafted
debris occurs just prior to the Middle Miocene deep-sea hiatus in Lower (?) Miocene sediments (Fig. 2; Hollister et al., 1976). The Middle Miocene
J. P. McGinnis et ul. : Marine Geology 141 (IYY7) YI-IVY
104
coincides with the oldest glacial unconformity (Mid-Miocene) on the continental Middle Miocene (Bart and Anderson, 1995). Near the end of the glacial maximum and subsequent retreat of the ice (Fig. 6, time e), subglacial meltwater streams may become an important process governing sediment transport to the continental rise. During this stage, sediment-laden meltwater introduced to the slope and into the
hiatus marks the transition from foraminifer- to radiolarian/diatom-dominated assemblages. This marked change in assemblages is interpreted as evidence for deepening of the CCD, and possibly a major shift in oceanic conditions (Tucholke and Houtz, 1976). During this period, the ice sheet may have remained at or near the outer continental shelf severely limiting terrigenous input to the basin. The timing for this deep-sea hiatus (Fig. 8)
=P
40
?
2?
‘P
Southern Antarctic Peninsula S. - Ttla Age of
ocemic
castat bse
T&
s.Add -
ofslope
openiylof
N.
N.- Add
rlr*e
paugetodeep circulation: ETUUnconrDnnny Trmtion from dcaraus to
-323-322-
silians reds.
DSDP323 & 325
d0fp-s
hiatuses
OSDP3251323 firstmajori&x of IUD
-325-
32-
-325-
_?_
Dsw322/323/325 Nlmmw&dratBT
Fig. 8. Oxygen isotopic record from Miller et al. ( 1987), tied to the major glacial and deep oceanic events predicted for the southern Antarctic Peninsula during the Cenozoic. Brackets indicate related time range based on DSDP results, and seismic data both on the shelf and rise. We tentatively correlate the regional deep-sea erosional unconformity (ETU), identified at the base of the Tula sediment deposits, to reflect the opening of Drake Passage to bottom waters at Oligocene/Miocene boundary time. The onset of canyon cutting across the continental rise occurred soon after the formation of the ETU surface.
J. P. McGinnis
et al. I Marine
deep canyons transports clays, silts, and finegrained sands to the rise (Fig. 7b). Sediments were likely delivered intermittently to the sediment mounds through the large glacial troughs which cross the continental shelf (Fig. 2; Bart and Anderson, 1995). The Antarctic Peninsula reflects the most temperate climatic conditions observed in the Antarctic, and may at various times have experienced conditions similar to those described for temperate glacial environments. In general, the present contribution of subglacial runoff is low around much of the Antarctic continent (e.g., Jacobs et al., 1992). Oceanographic measurements in Marguerite Bay indicate that warm CPDW flows across the continental shelf beneath the George VI Ice Shelf (Fig. 2; Potter and Paren, 1985; Jacobs et al., 1992). This warm water mass induces melting at the base of this ice sheet presentday (Pearson and Rose, 1983; Potter and Paren, 1985), and sediment cores across the shelf document meltwater-related glacial-marine facies in the form of diatomaceous and terrigenous muds (Kennedy and Anderson, 1989; Anderson et al., 1992; Pope and Anderson, 1992). Basal meltwater flow has therefore been documented beneath the George VI Ice Shelf in Marguerite Bay, and could reasonably be expected beneath other regions of the ice shelf. In contrast to the strata1 packages which depict deposition from a line source (Larter and Cunningham, 1993), a series of ‘point source’ depocenters develop across the rise. Our limited data cannot discern whether or not the large canyon systems extend landward onto and across the continental slope. At present, we have no data which document pronounced canyon cutting into the slope and across the distal shelf. Large troughs, however, are identified across the central and inner regions of the continental shelf (Fig. 2; Bart and Anderson, 1995). Extrapolated trends of these shelf canyons towards the outer shelf correspond to the trends of the canyons across the upper rise; we infer therefore that these canyon systems are likely related. The troughs have supplied significant volumes of sediment to the upper rise in the past, most likely during periods of significant glacial advance and the associated initial retreat of the expanded ice sheet. The deep glacial troughs
Geology 141 (1997)
91-109
105
(Fig. 2; > 1000 m in regions) that cross the shelf may also serve to focus meltwater runoff to the outer continental shelf, thereby promoting the point source development characterizing the large sediment mounds developed across the rise. The presence of the thick amalgamation of overbank deposits preserved throughout the stratigraphic record within the Tula deposit support our contention of an intermittent link between the troughs that cross the shelf and canyons that dissect the slope and rise. The introduction of increased meltwater to the slope should be reflected by a significant increase in channel-levee or canyon-overbank development across the continental rise during these periods. Nevertheless, the increased meltwater discharge should not result in downcutting and the formation of incised valleys because of the shelf physiography. Regrading of glaciofluvial networks in response to a baselevel fall is critically dependent on the pre-existing physiography of the fluvial system with respect to the previously submerged continental shelf. For example, if the slope of the exposed shelf is less than that of the glaciofluvial system, aggradation will occur in the lower reaches of the system. Conversely, if the slope is greater, then erosion will occur at the change in slope and systematically migrate upstream. If the previously submerged slope is equal to that of the glaciofluvial system, as is the case for large portions of the shelf in this region, then neither aggradation or erosional downcutting will occur. Sediments deposited across the rise at this time would be expected to consist primarily of fine-grained terrigenous clays and silty clays, interspersed with fine-grained sands as overflow bank deposits, and coarser material within the canyons. Seismic reflection data across the continental rise support our interpretation that channel bank formation occurs at the time of glacial maxima or initial ice retreat. Reflection profile SAP-8 shows a sediment package characterized by highly diffractive, chaotic reflectors directly overlain by well-developed, acoustically laminated overbank deposits associated with a canyon (Fig. 9). The chaotic facies unit probably represents debris flows sourced from the outer continental shelf and slope. This facies unit can be correlated across the lower
J. P. McGinnis
106
i
3
PI ul. ;’ Marine
Geology
141 (1997)
9/-
109
J.P. McGinnis et al. /Marine
slope to the steeply dipping, prograding seismic unit, which suggests that the chaotic unit was deposited when the ice advanced to and across the outer shelf edge. The canyon/overbank system developed after deposition of the chaotic unit and onlaps the debris flows when approaching the lower slope. The overbank deposits thin updip across the upper rise and towards the base of the slope; stratigraphically they overlie the steeply dipping prograding facies unit. The stratigraphic positioning of canyon/overbank system suggests that the canyon and its associated deposits formed after the ice had advanced across the outer portion of the shelf, either during the glacial maximum or during the initial retreat of the ice from the shelf. In some regions, sediments released during retreat of the ice accumulate along the outer continental shelf. As the outer shelf is exposed, bottom currents may be reestablished with velocities sufficient to transport fine-grained sediments along the shelf and into the glacial troughs (e.g., Singer and Anderson, 1984; Anderson and Smith, 1989). Along the Antarctic Peninsula, however, bottom currents do not appear prevalent over the shelf during the last glacial retreat, as indicated by the paucity of residual glacial-marine sands deposited there (Pope and Anderson, 1992). This condition implies that only a minor fraction of the sediments deposited into the Bellingshausen basin was likely derived from sources lateral to the canyons. Therefore, each canyon/overbank system was probably associated with a single drainage area and a relatively localized source. Sediment cores raised from the Bellingshausen lower rise and abyssal plain appear to document a restricted source locale for each of the major canyons (Baegi, 1985). Wright et al. (1983) and Baegi (1985) sampled four large depocenters associated with each major canyon crossing the continental rise. Mineralogic studies of the facies suggest that each of these depositional units portrays a distinct petrographic provenance. These results and the presence of coarse sands and gravels in some of the sampled turbidites, prompted Baegi (1985) to conclude that these sedimentary deposits were derived from a point source. Wright and Anderson (1982) have also suggested that coarse-grained sands and gravels recovered from cores on the Weddell Fan
Geology 141 (1997) 91-109
107
were delivered through canyons during periods of increased subglacial meltwater discharge.
5. Conclusions Across the Antarctic Peninsula continental rise increased canyon cutting occurred immediately after the formation of a prominent regional deepsea erosional unconformity. A tentative age of Late Oligocene to Early Miocene is assigned for the formation of this deep-sea erosional unconformity. This age appears to be consistent with a rapid transition to dominantly glacial conditions across the southern Antarctic Peninsula as indicated by the first influx of ice-rafted debris to the Bellingshausen basin and a shift from calcareousto siliceous-dominated assemblages (Hollister et al., 1976). This stratigraphic relationship suggests that the onset of highly fluctuating glacial conditions across the shelf began soon after the onset of intensified bottom water circulation along this portion of the margin. This change in depositional environment to one with an increased downslope influence may reflect increased sedimentation rates related to the onset of glacial conditions across the continental shelf at this time. The apparently rapid transition from an erosional environment to one of canyon/overbank formation suggests that a rapid surge of grounded ice breached the continental shelf soon after the onset of intensified bottom water circulation along the southern Antarctic Peninsula continental rise. The advent of dominantly glacial conditions may correspond to the invigoration of bottom currents along the margin; such currents were made possible by the Tertiary separation of the Antarctic Peninsula from South America and the subsequent opening of Drake Passage to cold bottom waters (Tucholke, 1977; Barker and Burrell, 1977). The development of this passage provided an avenue, for the first time for deep circumpolar currents around the Antarctic continent (Tucholke, 1977). The flow of these currents are regarded by many investigators as the critical event influencing Cenozoic global climate change and ocean circulation patterns that lead to the climatic isolation of Antarctica and the establishment of dominantly
108
.I. P. McGinnis et ul. ! Murinr Geology 141 11997) 91-109
glacial conditions there throughout the Neogene (Kennett and Shackleton, 1976; Kennett, 1977; Robin, 1988). The Tula sediment mound is comprised of an amalgamation of at least 5 -6 canyon/overbank systems which reflect various ‘point source’ contributions. Since the Antarctic shelf is so deep (>400 m) and dips in a landward direction, the transport of significant volumes of sediment to the basin must have occurred during a relatively brief period of time when the ice was near the shelf edge. Large canyons formed across the continental rise and thick overbank deposits related to these canyon systems formed either during or immediately after glacial maxima, when significant volumes of sediment could be delivered to the continental slope and rise. These canyon overbank deposits, comprised of clays, silts, and fine-grained sands, may be related to the introduction of meltwaters to the outer shelf and slope. Understanding how sediment is delivered to the continental slope and rise is critical to understanding the impact that glacial-interglacial climatic cycles played in controlling deep-sea depositional processes along the Antarctic continental margins. The southern Antarctic Peninsula margin may provide new opportunities to examine the impact of Antarctic paleo-circulation and its glacially modulated pulses on deep-sea sedimentary processes. The sediment mounds along the Antarctic Peninsula with their relatively high sedimentation rates offer some insight into how depositional environments may have varied along this and other high-latitude continental margins during periods of major tectonic and climatic change.
Acknowledgements We would like to acknowledge the University of Texas shipboard science party and the officers. crew, and technical staff of the R/V Ewing. NSF Grant OPP89-17322 supported both the Ewing based field-work and the subsequent data interpretation. The authors thank John Anderson and Lionel Carter whose thoughtful comments and constructive suggestions greatly improved this paper. Mary Ann Luisi and Juliet Malin assisted
in preparing the manuscript and figures. This is Lamont-Doherty Earth Observatory Contribution number 5641.
References Anderson, J.B., Smith, M.J., 1989. Formation of modem sandrich facies by marine currents on the Antarctic continental shelf. GCSSEPM Found. 7th Annu. Res. Conf. Proc., pp. 41-52. Anderson, J.B., Wright, R., Andrews, B.. 1986. Weddell Fan and associated abyssal plain, Antarctica: Morphology, sediment processes, and factors influencing sediment supply. Geo-Mar. Lett. 6, 121-129. Anderson, J.B., Kennedy, D.S., Smith, M.J., Domack, E.W., 1992. Sedimentary facies associated with Antarctica’s floating ice masses. In: Anderson, J.B., Ashley, G.M. (Eds.), Glacial Marine Sedimentation: Paleociimatic Significance. Geol. Sot. Am. Spec. Pap. 261, I-25. Baegi, M., 1985. Turbidite Deposition on the Bellingshausen Abyssal Plain: Sedimentologic Implications. Unpublished Master Thesis, Rice University, Houston, TX, 109 pp. Barker, P.F., 1982. The Cenozoic subduction history of the Pacific margin of the Antarctic Peninsula: ridge crest-trench interactions. J. Geol. Sot. London 139, 787-801. Barker, P.F.. Burrell. J., 1977. The opening of Drake Passage. Mar. Geol. 25, 15-34. Bart, P.J., Anderson, J.B., Seismic record of glacial events affecting the Pacific margin of the northwestern Antarctic Peninsula. In: Cooper, A.K., Barker, P.F., Brancolini, G. ( Eds.). Geology and Seismic Stratigraphy of the Antarctic Margin. 1995. Am. Geophys. Union, Antarct. Res. Ser. 68. 74-95. Bartek, L.R., Vail, P.R.. Anderson, J.B., Emmet, P.A., Wu, S., 199 1. Effect of Cenozoic ice sheet fluctuations in Antarctica on the stratigraphic signature of the Neogene. J. Geophys. Res. 96. 6753-6778. Drewry, D.J., Cooper, A.P.R., 1981. Processes and models of Antarctic glaciomarine sedimentation. Ann. Glacial. 2, 117 122. Dunbar. R.B., Anderson, J.B., Domack, E.W., Jacobs, S.S.. 1985. Oceanographic influences on sedimentation along the Antarctic continental shelf. In: Jacobs, S.S. (Ed.), Oceanology of the Antarctic Continental Shelf. Am. Geophys. Union, Antarct. Res. Ser. 43. 291-312. Goodell. H.G., 1968. Marine Geology of the Drake Passage, Scotia Sea, and South Sandwich Trench. Contrib. 7, Antarct. Res. Facil., Sedimentol. Res. Lab., Florida State Univ., Tallahassee, 277 pp. Haugland, K., Kristoffersen, Y., Velde, A., 1985. Seismic investigations in the Weddell Sea embayment. Tectonophysics 114, 293-313. Hayes, D.E. (Ed.), 1991. Marine Geological and Geophysical Atlas of the Circum-Antarctic to 30”s. Am. Geophys. Union, Antarct. Res. Ser. 54.
J. P. McGinnis et al. I Marine Geology 141 (1997) 91-109
Herron, E.M., Tucholke, B.E., 1976. Sea-floor magnetic patterns and basement structure in the southeastern Pacific. Init. Rep. DSDP 35, 263-278. Hollister, C.D., Craddock, C., et al., 1976. Init. Rep. DSDP 35, 930 pp. Jacobs, S.S., Amos, A.F., Bruchhausen, P.M., 1970. Ross Sea oceanography and Antarctic bottom water formation. DeepSea Res. 17, 51-70. Jacobs, S.S., Helmer, H.H., Doake, C.S.M., Jenkins, A., Frolich, R.M., 1992. Melting of ice shelves and the mass balance of Antarctica. J. Glacial. 38 (130), 375-387. Kennedy, D.S., Anderson, J.B., 1989. Quaternary glacial history of Marguerite Bay, Antarctic Peninsula. Quat. Res. 186, 255-276. Kennett, J.P., 1977. Cenozoic evolution of Antarctic glaciation, the Circum-Antarctic Ocean, and their impact on global paleoceanography. J. Geophys. Res. 82, 3843-3860. Kennett, J.P., Shackleton, N.J., 1976. Oxygen isotopic evidence for the development of the psychrosphere 38 Myr ago. Nature 260, 513-515. Larter, R.D., Barker, P.F., 1991. Neogene interaction of tectonic and glacial processes at the Pacific margin of the Antarctic Peninsula. In: MacDonald, DIM. (Ed.), Sedimentation, Tectonics, and Eustasy. Int. Assoc. Sedimentol. Spec. Publ. 12, 165-186. Latter, R.D., Cunningham, A.P., 1993. The depositional pattern and distribution of glacial/interglacial sequences on the Antarctic Peninsula Pacific Margin. Mar. Geol. 109, 203-219. McGinnis, J.P., Hayes, D.E., The roles of downslope and alongslope depositional processes: Southern Antarctic Peninsula continental rise. In: Cooper, A.K., Barker, P.F., Brancolini, G. (Eds.), Geology and Seismic Stratigraphy of the Antarctic Margin. 1995. Am. Geophys. Union, Antarct. Res. Ser. 68, 141-156. Miller, K.G., Fairbanks, R.G., Mountain, G.S., 1987. Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion. Paleoceanography 2 (1) l-19. Normak, W.R., 1978. Fan valleys, channels, and depositional lobes on modem submarine fans: Characters for recognition of sandy turbidite environments. AAPG Bull. 62, 912-931. Pearson, M.R., Rose, I.H., 1983. Dynamics of George VI ice shelf. Br. Antarct. Surv. Bull. 52, 205-220.
109
Pelayo, A.M., Wiens, D.A., 1989. Seismotectonics and relative plate motions in the Scotia Sea region. J. Geophys. Res. 94 (B6), 7293-7320. Pope, P.G., Anderson, J.B., 1992. Late Quaternary glacial history of the northern Antarctic Peninsula’s western continental shelf: Evidence from the marine record. Antarct. Res. Ser. 57, 63391. Potter, J.R., Paren, J.G., 1985. “Interaction between ice shelf and ocean in George VI Sound, Antarctica. In: Jacobs, S.S. (Ed.), Oceanology of the Antarctic continental shelf. Am. Geophys. Union, Antarct. Res. Ser. 43, 35-58. Rebesco, M., Larter, R.D., Barker, P.F., Camerlenghi, A., Vanneste, L.E., 1994. The history of sedimentation on the continental rise west of the Antarctic Peninsula. Terra Antarct. 1 (2), 277-279. Robin, G. de Q., 1988. The Antarctic ice sheet, its history and response to sea level and climatic changes over the past 100 million years. Palaeogeogr., Palaeoclimatol., Palaeoecol. 67, 31-50. Shanmugam, G., Moiola, R.J., 1982. Eustatic control of turbidites and winnowed turbidites. Geology 10, 231-235. Singer, J.K., Anderson, J.B., 1984. Use of total grain-size distributions to define bed erosion and transport for poorly sorted sediment undergoing simulated bioturbation. Mar. Geol. 57, 335-359. Tucholke, B.E., 1977. Sedimentation processes and acoustic stratigraphy in the Bellingshausen Basin. Mar. Geol. 25, 209-230. Tucholke, B.E., Houtz, R.E., 1976. Sedimentary framework of the Bellingshausen basin from seismic profiler data. Init. Rep. DSDP 35, 197-228. Uchupi, E., Swift, S.A., 1991. Plio-Pleistocene slope construction off western Nova Scotia, Canada Cuad. Geol. Iber. 15, 15-35. Wright, R., Anderson, J.B., 1982. The importance of sediment gravity flow to sediment transport and sorting in a glacial marine environment: Eastern Weddell Sea, Antarctica. Geol. Sot. Am. Bull. 93, 951-963. Wright, R., Anderson, J.B., Fisco, P.P., 1983. Distribution and association of sediment gravity flow deposits and glacial/ glacial-marine sediments around the continental margin of Antarctica. In: Molnia, B.F. (Ed.), Glacial-Marine Sedimentation. Plenum, New York, pp. 265-300.
