Depositional Environments for Strata Cored in CRP-3 (Cape Roberts Project), Victoria Land Basin, Antarctica: Palaeoglaciological and Palaeoclimatological Inferences

Cape Roberts Project drill core 3 (CRP-3) was obtained from Roberts ridge, a sea-floor high located at 77oS, 12 km offshore from Cape Roberts in western McMurdo Sound, Antarctica. The recovered core is about 939 m long and comprises strata dated as being early Oligocene (possibly latest Eocene) in age, resting unconformably on ~116 m of basement rocks consisting of Palaeozoic Beacon Supergroup sediments. The core includes ten facies commonly occurring in five major associations that are repeated in particular sequences throughout the core and which are interpreted as representing different depositional environments through time. Depositional systems inferred to be represented in the succession include: outer shelf, inner shelf, nearshore to shoreface each under iceberg influence, deltaic and/or grounding-line fan, and ice proximal-ice marginal-subglacial (mass flow/rainout diamictite/subglacial till) singly or in combination. The record is taken to represent the initial talus/alluvial fan setting of a glaciated rift margin adjacent to the block-uplifted Transantarctic Mountains. Development of a deltaic succession upcore was probably associated with the formation of palaeo-Mackay valley with temperate glaciers in its headwaters. At that stage glaciation was intense enough to support glaciers ending in the sea elsewhere along the coast, but a local glacier was fluctuating down to the sea by the time the youngest part of CRP-3 was being deposited. Changes in palaeoenvironmental interpretations in this youngest part of the core are used to estimate relative glacial proximity to the drillsite through time. These inferred glacial fluctuations are compared with the global δ18O and Mg/Ca curves to evaluate the potential of glacial fluctuations on Antarctica for influencing these records of global change. Although the comparisons are tentative at present, the records do have similarities, but there are also some differences that require further evaluation. INTRODUCTION AND REGIONAL SETTING The Cape Roberts Project is an international cooperative drilling programme that was originally designed to recover continuous drill core from strata between about 30 and 100 Ma from western McMurdo Sound, Antarctica. The main aim of the project is to study the poorly constrained tectonic and climatic history of the region for this period of time. During the 1999 austral summer the third hole of the project, CRP-3, was drilled in 295 m of water, 12 km off Cape Roberts (Cape Roberts Science Team, 2000). CRP-3 was cored to 939 mbsf (metres below sea floor) with a 97% core recovery and terminated in strata thought to be of Devonian age sitting unconformably below strata of earliest Oligocene to latest Eocene age. The drillsite was located on a sea floor high, Roberts ridge, which is a tectonic horst thought to have been rotated perhaps during and after Miocene time (see Cape Roberts Science Team, 2000, Figs. 7.7 and 7.8). Roberts ridge rises 500 m from the surface of the graben infill to the west between it and the present coast. To the north of Roberts ridge is a deep, sinuous, east-west trending sea floor trough, the Mackay Sea Valley. This is over 900 m deep and thought to have been eroded by an expanded Mackay Glacier. This glacier is a major outlet for the East Antarctic Ice Sheet and feeds into Granite Harbour just north of Cape Roberts. Like the Ferrar Valley 70 km to the south (cf. Barrett, 1989; Barrett & Hambrey, 1992), it is likely that the Mackay system has been a valley and palaeofjord since at least the mid-Oligocene times with the palaeo-Mackay Glacier advancing and receding within its trough. It is also known that several times during the Cenozoic Era *Corresponding author ([email protected]) R.D. Powell et al. 208 grounded ice expanded in the Ross Sea to a position well north of Roberts ridge (Brancolini et al., 1995; Licht et al., 1996). This ice may have eroded younger strata from the top of the ridge (Cape Roberts Science Team, 1998, p. 4). Cape Roberts Project drillhole 1 (CRP-1) was drilled on Roberts ridge up-dip from CRP-2/2A and 3. The recovered core was about 147 m long, with the upper 43.55 mbsf dated as Quaternary and the older part of the sequence early Miocene (Roberts et al., 1998). This core includes nine facies: sandy diamict, muddy diamict, gravel/conglomerate, rubble/breccia, graded poorly sorted sand(stone), better sorted stratified sand(stone), mud(stone), clay(stone) and carbonate (Cape Roberts Science Team, 1998). Seven depositional systems were recognised on the basis of the facies: offshore shelf, ice protected/below wave-base; prodeltaic/offshore shelf; delta front/sandy shelf; ice contact and ice proximal, mass flow and submarine, fluvial efflux system; ice-contact and ice proximal, mass flow system; subglacial till/rainout diamict/debris flow diamicts singly or in combination; and a carbonaterich shelf bank (Powell et al., 1998). The Quaternary section was interpreted to represent deposition on a polar shelf with two or three glacial fluctuations, and the Quaternary carbonate unit was thought to indicate a period of ice sheet retreat. In contrast, the early Miocene section was thought to represent deposition from polythermal glacial systems. The older early Miocene section was glacially dominated whereas the younger part was much less so. CRP-2/2A was also drilled on Roberts ridge updip from CRP-3. CRP-2 was cored from 5 to 57 mbsf and CRP-2A was a minor drilling deviation at the same site, reaching down to 624 mbsf which terminated in early Oligocene strata (about 31 Ma, Wilson et al., 2000). The core was described as having twelve facies commonly occurring in associations that are repeated in particular facies sequences throughout the core and which were interpreted as representing different depositional environments through time (Cape Roberts Science Team, 1999). Depositional systems inferred to be represented in the succession included: outer shelf, inner shelf and nearshore to shoreface under iceberg influence, deltaic and/or grounding-line fan, and ice proximal-ice marginal-subglacial (mass flow/rainout diamictite/subglacial till) singly or in combination (Powell et al., 2000). The CRP-2/2A succession was interpreted in terms of deposition in glacimarine and coastal marine environments by a combination of tractional currents, fall out from suspension, sediment gravity flows and mass flows, rain-out from floating, glacial ice and deposition and redeposition in subglacial positions. By comparisons with modern glacimarine settings, this facies analysis showed that the amount of melt-water associated with the glaciers probably decreased from Oligocene time through the Miocene. In terms of comparative modern settings the data appear to agree with CRP-1 for the Miocene where the setting is thought most comparable with polythermal glaciers in the sub-Arctic (Powell et al., 1998). The trends of increasing evidence of meltwater and increasing rates of sedimentation down-core are used to infer progressively warmer temperatures and more temperate glaciation. The extreme endmember of fully temperate glaciation, as is found in Alaska, Iceland and Chile, appears to have been approached by strata lower in the core, based on proportions of preserved facies (Powell et al., 2000). GENERAL STRATIGRAPHY AND LITHOFACIES The Cenozoic strata cored in CRP-3 have been described lithologically and divided into 15 lithostratigraphic units and 34 subunits (Cape Roberts Science Team, 2000, Fig. 3.1, p. 59). They are thought to represent more or less continuous sediment accumulation with numerous small time breaks in the early Oligocene, possibly extending into the latest Eocene (31 to ca. 34 Ma; Hannah et al., this volume) (Fig. 1). Ten recurrent lithofacies are recognised within the core and are defined using lithologies or associations of lithologies, bedding contacts and bed thicknesses, texture, sedimentary structures, fabric and colour. The lithofacies scheme used for CRP-3 follows that for CRP2/2A with the exception of two facies, volcaniclastics and mudstone breccia, that do not occur in CRP-3. Although the same scheme is followed, modifications to the descriptions have been made because some characteristics are particular to CRP-3. The 10 lithofacies, which are based primarily on the visual core descriptions reported by the Cape Roberts Science Team (2000), are presented in table 1 and appendix 1, along with our palaeoenvironmental interpretations. The reader is referred to photographs of each facies and particular features of note in the CRP-3 Initial Report (Cape Roberts Science Team, 2000, pp. 68-75). FACIES SEQUENCES AND DEPOSITIONAL ENVIRONMENTS THROUGH TIME The facies outlined above have common associations throughout the core. A combination of individual and associations of facies in vertical sequences and some particularly distinctive sedimentological or biological characteristics are used to interpret depositional environments up the core (Fig. 1; Tab. 1). This analysis is a synthesis, and attempts to keep groupings and facies sequences to a minimum; alternative interpretations may be possible in some instances and are discussed in the text below. The alternative interpretations may be resolved in Depositional Environments for Strata Cored in CRP-3 209 future when other data, such as from palaeoecology, are also considered. The sequences are interpreted as representing particular settings which, when combined, define broad sedimentary environments and changes in environments through time. Some apparent dislocations in what could be predicted as a logical succession according to the principles of Walthers’ Law, occur in parts of the core between the sequences Fig. 1 Graphic lithofacies log of CRP-3, showing interpreted lithofacies associations with the depth (metres below sea floor) column, summary sedimentary structures (A), general facies with mean particle size profile (B), facies codes for facies 1 through 10 (C), distribution of number of clasts per metre ranging from 0 to over 100 (D), inferred glacial proximity (m marine, d distal glacimarine, p proximal glacimarine, i/bice contact/subglacial) (E), and interpreted depositional system and inferred palaeoenvironment (F). Note that facies in the LSU 2.1 (83.10 to 95.48 mbsf) interval are modified from the original log (Cape Roberts Science Team, 2000) following particle size analyses of Barrett (this volume). R.D. Powell et al. 210 of interpreted facies associations. The dislocations may be real and indicate intervals of erosion, such as by a glacier, or they may represent extremely rapid switches in depositional processes, as is common in the inferred environments. The percentage of each facies up the core was tabulated by subdividing the succession into its lithostratigraphic units (LSUs), in order to evaluate trends with time (Tab. 2). A note of caution is that the record is likely to have been deposited under very high sediment accumulation rates in most intervals (see discussion below) and given the time control on the succession, much of the record must be missing. We have no way of estimating the proportion of each facies in each time interval that has been lost. Thus the percentages used here are probably biased toward deeper water and/or more ice-distal deposits, which have a greater likelihood of being preserved. The oldest preserved record (LSU 15.3-15.1) above the Beacon Sandstone is dominated by coarse clastic debris interpreted as terrestrial talus. This is followed immediately by conglomerate supported by a matrix dominated by mudstone (LSU 14.1), which is inferred to represent deposition on a fan-delta with sufficient marine or lacustrine influence to allow quiet water sedimentation and settling of fines. Through a transition (LSU 13.2) the succession is then dominated by sandstones with minor conglomerates (LSU 13.1-11.1) that are most likely fluvial-deltaic associations. At this time, glaciers may have been becoming established locally as a sediment source, but they were further developed elsewhere because they produced icebergs to raft lonestones to the site. Thicker mud intervals are first preserved in LSU 12.2, but are more common in the LSUs 10.1 and younger, indicating the preservation of probably more common offshore conditions. Between LSU 10.1 and 3.1 there are repeated fining-upwards intervals from conglomerate through sandstone to mudstone facies. These are interpreted as having a source from glacial meltwater discharges and deposited in shallow marine settings, mostly above wave base and most likely in a deltaic system. Local glaciers were probably entering the sea during times represented by LSU 6.1 and younger, based on the poorly sorted character of the facies and presence of diamictites. Following and including the time that LSU 2.2 accumulated, glaciers had a strong presence in the area as reflected by thick diamictite units in a commonly mud-rich, marine section. Using the models of glacimarine sedimentation (e.g. Dowdeswell, 1996; Powell & Alley, 1997; Dowdeswell et al., 1998) as a guide, the large amount of sorted sediment introduced by meltwater, the common deltaic systems, the rapid sediment accumulation rates and a dominance of gravelly mudstones/muddy sandstones over diamict, all point to a warm glacial system present in the area, being temperate or perhaps the very warm end of the polythermal glacier spectrum. Following these general trends in facies, five Facies number and name Key sedimentological characteristics Depositional process interpretation Key interpretation criteria 1 Mudstone massive, often sandy local laminae common lonestones locally brecciated marine macroand microfossils hemipelagic suspension settling rainout from ice rafting may be modified by other processes brecciated by tectonism or glacial tectonism fine-grained character isolated clasts marine fossils 2 Interstratified sandstone and mudstone sandstones on sharp contacts sandstones grade up, often to mudstones massive and amalgamated beds planar stratified local ripple cross-lamination some normal, local reverse grading dispersed to abundant clasts marine macroand microfossils range of marine processes: lowto moderatedensity sediment gravity flow deposition; combined wave and current action rapid deposition and resedimentation sandstone/mudstone association style of internal stratification and grading marine fossils 3 Poorly sorted (muddy) very fine to coarse sandstone various poorly sorted sandstones locally massive and amalgamated locally planar laminated and bedded normal grading, local reverse local ripple cross-lamination local soft-sediment deformation, boudinage local dispersed clasts grading to matrix-supported conglomerate marine macroand microfossils mediumto high-density sediment gravity flow deposition very fine to fine sandstones may be from settling from turbid plumes with high sediment concentrations may be massive due to depositional processes or mixing by bioturbation, freeze/thaw, loading style of internal stratification and grading degree of sorting marine fossils 4 Moderately to well sorted, stratified fine sandstone local low angle cross-bedding and cross-lamination locally planar, thin bedded to laminated possible HCS quartz rich, local coal laminae locally with dark mudstone, bituminous penecontemporaneous soft-sediment deformation marine macroand microfossils dilute tractional currents (within or about wave base to shoreface) style of internal stratification particle size and sorting marine fossils 5 Moderately to well sorted, stratified or massive, fine to coarse sandstone mostly medium-grained, locally fine or coarse planarto cross-stratified locally massive and amalgamated dispersed to abundant clasts local gravelly layers at base weak to moderate bioturbation marine fossils marine currents/wave influence (perhaps shoreface) local erosion with hiatuses rainout from iceberg rafting particle size and sorting style of internal stratification bioturbation marine fossils Tab. 1 Summary table of facies characteristics and their interpretations of core CRP-3*. *Notes in addition to those presented in the Initial Reports volume (Cape Roberts Science Team, 2000) regarding interpretation of these facies are presented here in the Appendix. Depositional Environments for Strata Cored in CRP-3 211 facies associations have been defined in upward succession (for details of interpretation see Cape Roberts Science Team (2000; pp. 187-197) and in the discussion section below): 1. monomictic breccia and conglomerate, derived from the Beacon Supergroup (823.11-822.88 mbsf) interpreted as probably being talus immediately above the unconformity due to clast composition and angularity. 2. clast-supported conglomerate and minor sandstone (822.88-789.77 mbsf) interpreted as grading upward from facies association 1 into a high gradient fluvial system capable of transporting boulders of greater than 1 m initially with a talus contribution. Up-core from about 804 mbsf, mud becomes a major matrix component and is taken to possibly represent a transition from terrestrial to marine conditions. 3. muddy sandstone with subordinate conglomerate (789.77-~580 mbsf) are interpreted as sediment gravity flow deposits mainly of debris flow and high-density turbidity current origin. They are thought to have been deposited in a deltaic setting in varying water depths but often above at least storm wave-base. Iceberg rafting over the delta probably indicates that the delta itself was fed by glacial outwash streams. 4. clean sandstone with subordinate conglomerate (~580-378.36 mbsf) which due to their better sorting, are thought to have been deposited in shallower water than facies association 3, though they are still marine, and likely to be delta-front deposits. Their cleanness may also be a function of the decreasing proximity of the glacier source or by an increase in sorting due to wave action (Barrett, this volume). 5. muddy sandstone and mudstone, with subordinate conglomerate and diamictite (378.36-0.00 mbsf) are similar to associations found in younger CRP cores and are interpreted as being of temperate glacimarine origin commonly associated with their