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The interpretation of massive rain-out and debris-flow diamictites from the glacial marine environment
Abstract
Glacial deposits provide a long-term record of climate and sea level changes on Earth. Detailed study of sedimentary rocks deposited during and immediately after glacial episodes is paramount to accurate palaeoclimatic reconstructions and for our understanding of global climatic and eustatic changes. This book presents new information and interpretations of the ancient glacial record, looking in particular at the Late Proterozoic and Late Paleozoic eras. The influence of global tectonics on the origins and distribution of ice masses and the character of glacial deposits through geologic time is emphasised. Sequence stratigraphic techniques are applied to glaciogenic successions, and explanations for possible low-latitude glaciation during the Late Proterozoic era and the association of carbonate deposits with glaciogenic rocks are put forward. Early interglacial conditions, represented by dark grey mudrocks and ice keel scour features are discussed. These studies, from key workers in International Geological Correlation Program Project 260, will aid the understanding of the Earth's climatic history.
... Interpretation. Massive diamictites with faceted/striated clasts may have multiple origins, including direct emplacement by a glacier, ice rafting and resedimentation of glacial material via debris flows (e.g., Visser, 1989Visser, , 1994Gama Jr. et al., 1992;Eyles et al., 1993). However, the association of massive diamictites, diamictites with deformational structures and allochthonous blocks in U1, suggests the origin of this unit as gravity flows, specifically slumps (e.g., Mottin et al., 2018;Valdez Buso et al., 2019;Rodrigues et al., 2020Rodrigues et al., , 2021. ...
... Interpretation. Massive diamictites can be formed by multiple processes, as a combination of settling of fines from meltwater plumes and rain-out, debris-flow, or a combination thereof (Visser, 1994;Isbell et al., 2021). Massive diamictites and facies with rafted blocks may be different parts of a continuum process of disaggregation and mixing within subaqueous mass flows. ...
... Clasts range from granule to boulder (max. size: 1.2 m), are commonly faceted, striated and bullet-shaped 5-15 m N/O Debris rain-out (mud, sand and oversized clasts), and/or resedimentation of glacial material through subaqueous debris flows, or a combination thereof (e.g., Gama Jr. et al., 1992;Eyles et al., 1993;Visser, 1989Visser, , 1994 Diamictite matrix characterized by mm-to cm-thick textural bands (muddy and sandy discontinuous layers) and dispersed granules to pebbles. Lenticular, wavy and flaser bedding facies represent distal parts of the mouth bar, formed by alternating bedload transport and suspension fallout. ...
The late Paleozoic glacial-to-postglacial turnover evolved complexly across Gondwana. Successions bearing
volcaniclastic material that can be radiometrically dated provide crucial information about the timing of those climate
events. The southernmost part of the Paraná Basin, for instance, has a high-precision geochronological framework.
The eastern sector of this basin (Paraná State and north of Santa Catarina State), however, lacks radiometric
ages, but conversely, has a more complete stratigraphic record, and paleontological information still poorly explored
for the purpose of biostratigraphic correlation. This work examines the glacial-to-postglacial interval in
the Paraná State, represented by the upper Itararé Group (Taciba Formation; glacial) and lower–middle Rio Bonito
Formation (postglacial). Sedimentological, paleontological and geochemical data from outcrops, cores and well
logs were used to decipher the timing, paleoclimatic and paleogeographic scenarios of this transition. The examined
succession comprises four stacked units (U1 to U4, from older to younger). Diamictite-dominated units (U1
and U3), here interpreted as consecutive glaciation–deglaciation events, are separated by non-glacial, continental
to shallow marine deposits, commonly bearing fossil plants and coal seams (U2). An important transgression
followed the first deglaciation,which is equivalent to the “Eurydesma transgression”, basedonthepresence ofmarine
invertebrates of the homonymous fauna in the Passinho Shale. U2 holds elements of two different floras,
i.e., Phyllotheca–Gangamopteris (P–G), predating the “Eurydesma transgression”, and Glossopteris–Brasilodendron
(G–B), above the transgression. Therefore, U2 is interpreted as an interglacial interval, once it records a climate improving
before the last glacial episode of U3, which is further supported by relatively high values of the Chemical
Index of Alteration (CIA). Deposits of U3, associated with a decrease in the value of the CIA, are unconformably
overlain by U4. The occurrence of coal-bearing postglacial facies (U4) associated with the G–B Flora, coincides
with an increase in the CIA values. Sediment transport was toward the SWin all units and in the same direction
it is observed an overall thinning of U1 and U3 and thickening of U2. The interglacial P–G Flora of the study area
correlates with postglacial southernmost floras, based on U–Pb CA-TIMS Asselian ages of tonsteins. Correlation
of the Eurydesma fauna-containing Passinho Shale with equivalent successions with high-precision age control
in southern Africa, allowed us to position both deglaciations of the Taciba Formation (U1 and U3) in the Asselian.
These findings suggest that U1 and U3 record two early Permian glacial episodes, with the younger one (U3)
disappearing southward. Accordingly, our results indicate that the glacial-to-postglacial turnoverwas diachronous
along the eastern belt of the Paraná Basin, being progressively older southward, considering that interglacial fossil
plant assemblages in the eastern margin correspond in time to postglacial assemblages farther south.
... The weakly stratified diamictite is likely formed in sedimentary environments characterized by relatively high sedimentation rate and persistent debris flow system. These environments may have resulted from various processes including the rain-out of clasts from floating glaciers and/ or melt water plumes and subsequent re-deposition through coherent debris flow (cf., Eyles et al., 1985;Visser, 1994;Powell and Domack, 2002), or from sloperelated subaqueous debris flows generated by slumping and sediment failure (Eyles and Eyles, 2000). Likewise, the stratified diamictite is formed under similar sedimentary environment with higher degree of dilution, causing a decrease in grain/water ratio and deposited from the coherent debris flow (Sohn et al., 1999). ...
... However, striated clasts, dropstone structure with bended laminae, and cold-water faunal assemblage reported in previous works are indicative of a glaciogenic sedimentary environment (Fan and Fang, 1992;Luo et al., 2018;Wang et al., 2001). Hence, we suggest that the overall poor sorting of these facies and the presence of lonestones in the massive sandy siltstone facies indicate a proximal glaciomarine environment (e.g., Visser, 1994;Isbell et al., 2013). Furthermore, without presence of glaciogenic deformation, we suggest that the weakly stratified diamictite and stratified diamictite facies were deposited by pro-glacial fan deposits in a form of coherent debris flow (Bussert, 2014). ...
The Baoshan Block was part of the eastern Cimmerian continent along the northeastern margin of Gondwana during the late Paleozoic, and preserves continuous sedimentary records during the apex of the Early Permian glaciation. Although much work has been carried out on biostratigraphy and paleogeography, detailed sedimentological study of the glacial records in the Baoshan Block has gained little attention, which hampers the understanding of depositional processes along the northeastern margin of Gondwana. Here, we conduct a high-resolution sedimentological study on the Lower Permian Dingjiazhai Formation of the Baoshan Block, with the aim of reconstructing the evolution of sedimentary environments. Six major lithological facies and three facies associations are recognized from the Dongshanpo section. The diamictite facies association is composed of weakly stratified diamictite, stratified diamictite, and massive sandy siltstone facies, which are formed by proglacial outwash debris flows and settling of melt-water plumes. The lenticular gravelly sandstone facies association consists of lenticular gravelly sandstone, thin-bedded siltstone, and homogenous silty mudstone facies, suggesting a distal subaqueous proglacial offshore setting with episodic melt water plumes. The bioclastic rudstone facies association comprises of bioclastic rudstone and floatstone, and thin-bedded siltstone facies, which suggest an offshore depositional environment punctuated by bioclastic gravity flows. Accordingly, we reconstructed a gradually changed sedimentary model responding to the Early Permian deglaciation in the Baoshan Block, which probably represents a typical evolution of sedimentary environments along the northeastern margin of Gondwana during the Early Permian.
... The glaciogenic interpretation was first published in 1870 (Hancox & Götz, 2014), generally accepted in 1898 (Sandberg, 1928), and basically has withstood critical comments and alternative geological interpretations. During the early investigations of the Dwyka Group, most sections were considered to have been deposited subglacially, but as more data accumulated, interpretations have become more complicated, including many and varied glacial advances and retreats, and recognising the presence of sediment gravity flows and rain-out deposits (Visser, 1986(Visser, , 1990(Visser, , 1994(Visser, , 1996(Visser, , 1997Visser et al., 1997aVisser et al., , 1997bIsbell et al., 2008;Dunlevey & Smith, 2011). ...
... Therefore, reinterpretation of the Dwyka Group diamictites may not affect the interpretation of any other Gondwana diamictites, as the glaciation at any rate is considered to be patchy or discontinuous during its long duration. Massive diamictites have been discovered to be often stratified and to be aprons, fans or debris flows and not deposited subglacially (Visser, 1994(Visser, , 1997Visser et al., 1997a;Huber et al., 2001;Haldorsen et al., 2001;Isbell et al., 2008;Dietrich & Hofmann, 2019;Johan Visser, pers. comm., 2020), and some authors believe that there are only very few places where there is subglacial basal/lodgement tillite (Visser, 1997;Isbell et al., 2008), while others are more likely to interpret diamictites as subglacially formed (e.g., Blignault & Theron, 2015). ...
The Gondwana Late Palaeozoic Ice Age is probably best represented by the Dwyka Group in South Africa. Striated and grooved surfaces or pavements are commonly considered to have formed subglacially, as are diamictites which have been interpreted as in-situ or reworked tillites. These interpretations were tested by investigation of outcrops in formerly well-studied areas, throughout South Africa. Detailed analyses have focused on striated surfaces/pavements and surface microtextures on quartz sand grains in diamictites. The sedimentological context of four pavements, interpreted to be glaciogenic, display features commonly associated with sediment gravity flows, rather than glaciation. A total of 4,271 quartz sand grains were subsampled from outcrops that are considered mainly to be tillites formed by continental glaciation. These grains, analysed by SEM, do not demonstrate the characteristic surface microtexture combinations of fracturing and irregular abrasion associated with Quaternary glacial deposits, but mainly a mix of surface microtextures associated with multicyclical grains. The Dwyka Group diamictites warrant reinterpretation as non-glacial sediment gravity flow deposits.
... the cores, which however are not always present. Massive diamictite could be also generated from suspension settling of the fine-grained component by meltwater plumes and iceberg rainout, and release of coarse-grained component from iceberg and floating ice in an icecontact glaciomarine environment (Eyles et al., 1985;Cowan and Powell, 1991;Dowdeswell et al., 1994;Visser, 1994;Smith and Andrews, 2000;Powell and Domack, 2002;Powell, 2005). Moreover, massive diamictite could be the result of subaqueous gravity flows (Powell and Molnia, 1989;Rodrigues et al., 2020;Isbell et al., 2021), and subglacial deposition (Dreimanis, 1989;Boulton, 1990;Evans et al., 2006). ...
... Diamictite, as shown for FA1, could be generated from various processes in different depositional environments. The presence of weak stratification, faint lamination, and mud wisps within the thick bodies of unstratified or weakly stratified diamictite reveals that deposition should be likely linked with high decantation rate from suspended sediments under the water column, rather than with mass transport processes (Bjørlykke et al., 1976;Visser, 1994Visser, , 1996Powell and Domack, 2002;Ives and Isbell, 2021), which cannot be completely ruled out and possibly may have sometimes played a minor role. Clasts long axis alignment could be the result of decantation from icebergs rainout, while it is also been recorded in subglacial tills (Benn, 1995;Hicock et al., 1996;Hooyer and Iverson, 2000;Carr and Rose, 2003;McKay et al., 2009). ...
The Late Paleozoic Ice Age (LPIA) is one of the coldest periods in Earth history, and led to the diachronous development of widespread ice centers across Gondwana in the Carboniferous and Permian. In Tasmanian Basin, located between northern Victoria Land (Antarctica) and Australia, the lowermost part of the Parmeener Supergroup (Late Carboniferous to Triassic), consisting of the Wynyard Tillite and its correlatives (Truro and Stockers tillites), recorded LPIA glacial sedimentation linked with ice covers that developed in the region. We carried out a detailed facies analysis of two drillcores which recovered glaciogenic sequences deposited in the Tasmanian Basin. Facies associations vary from possibly sub-glacial or ice-contact to ice-distal. Diamictite is the most common facies and its deposition is driven both by gravity and sediment remobilization processes and suspension settling with ice rafted debris accumulation. Mudstone layers, with and without dropstones, are interposed between diamictite intervals, recording ice-distal to non-glacial conditions respectively. Facies associations are indicative of subaqueous deposition in glacimarine environment. The glacial sequence stratigraphy approach was applied and glacial system tracts and bounding surfaces, which define glacial sequences, were identified. The stacking pattern of the facies associations allow us to demonstrate that the glacial sequences in the Tasmanian Basin recorded different phases of advance and retreat of the glacial front into the basin, at about the end of the main glacial phase.
... The Prince Albert Formation is divided into two units in the study area, a lower chestnut-brown unit and an upper blackish-brown unit. The lower unit contains pebble to small boulder sized concretions and beds of phosphatic, cherty siltstone (Visser, 1994a). ...
... The texture and geochemistry of calcite and phosphatic concretions presented above indicate that the concretions formed in a fresh, glacial melt water lake in the study area of the southwestern Karoo Basin. Deposition of massive diamictite reflects periods of maximum ice sheet extent when glacial till was deposited by subglacial ice sheet melting and by high density rain-out in a proximal environment (Visser, 1994a). We propose that the concretions formed in the massive diamictite deposits during periods of ice sheet retreat when large amounts of runoff supplied muddy glacial melt water and meteoric water to continental basins. ...
The upper Dwyka and lower Ecca Groups in the Karoo Basin of South Africa document the climatic and palaeoenvironmental changes associated with the final Permo-Carboniferous deglaciation of the Gondwana supercontinent. The depositional environments of these groups have, until recently, been interpreted on the basis of sedimentological and palaeontological evidence. Here we use the geochemistry of early diagenetic concretions septarian calcite concretions from the upper Dwyka Group and phosphatic chert concretions and beds from the lower Ecca Group to infer the depositional environment of these rocks in the southwestern Karoo Basin. delta18O values (7.8 to 8.90/00 SMOW) suggest that the calcite concretions precipitated from a mixture of meteoric and glacial melt waters rather than Permian seawater. delta13C values (- 15 to - 30/00 PDB) indicate that the carbon was derived from a mixture of craton-derived calcareous material and organic matter, bacterially degraded in the lower sulphate-reduction to upper methanogenesis zones during early burial diagenesis. The rare-earth element (REE) patterns, Sr concentrations and 87Sr/86Sr ratios (0.716 0.737) significantly greater than Permian seawater (0.708), together also support the interpretation that calcite and phosphatic concretions formed in glacial, fresh water sediments.
... Diamictite is associated with either glacial deposition or downslope sediment-gravity flow-e.g., debris flow (Eyles et al., 1983;Fairchild et al., 1989;Moncrieff and Hambrey, 1990;Link et al., 1994;Visser, 1994;Miller, 1996). Several features in the DCU suggest glacial influence during deposition (c.f. ...
... Massive diamictite is interpreted as either lodgment or rain-out till. Distinguishing between the two is difficult (Visser, 1994;Miller, 1996), and the facies must be considered in stratigraphic context. Where diamictite directly overlies exposed and/or striated basement, forming beds Ͻ1 m thick, lodgment of debris beneath grounded ice is the most likely explanation. ...
The 80-m-thick Egan Formation preserves sediment deposited during the younger of two episodes of glaciation recorded in the Neoproterozoic succession of the Kimberly region, northwestern Australia. Like many terminal Proterozoic glaciations recorded in Australia and elsewhere, the glacial strata of the Egan Formation are associated with carbonate rocks of likely warm-water affinity, but they are sedimentologically distinct from the marker "cap carbonate" horizons that overlie glacial strata in other Neoproterozoic successions. The carbonate strata comprise a wide range of facies indicative of shallow-water patch reef, shoal, and lagoonal deposition. Detailed facies analysis of the Egan Formation indicates interruption of the carbonate system by glaciation and subsequent resumption of warm-water conditions. This sedimentological analysis allows a reassessment of the regional stratigraphic correlations proposed for the Egan Formation, which is here considered to record a glacial event younger than the widespread Marinoan glaciation of central and South Australia and, therefore, a speculated third global glaciation in terminal Proterozoic time.
... Many of the clasts display long-axis dip orientations that exceed 20°. Although the lithofacies is described as massive, crude stratifi cation occurs and is identifi ed by faint bedding and by scattered clast and grain horizons (Visser, 1994). Mudstone units, less than 0.5 m thick, and rare single-layer boulder beds, which can be traced over several kilometers, also occur within the diamictite . ...
... In comparison, erosional contacts at the base of some of the overlying successions may indicate that grounded ice advanced into the basin (Visser, 1997a). Throughout much of the massive diamictite, crude stratifi cation, high-angle dips of the long axes of clasts (up to 30% of clasts have high AB-plane dips), and the occurrence of beds of dropstone-bearing mudstones are interpreted as having formed from subaqueous rain-out and sediment gravity fl ows (Visser, 1994Visser, , 1997aVisser, , 1997b ). Rain-out deposits are produced by a combination of suspension settling of fi nes from meltwater plumes and icebergs. ...
Pennsylvanian and Permian glacigenic deposits of the Dwyka Group occur within Karoo basins throughout southern Africa. The largest, the main Karoo Basin, evolved into a foreland basin during Dwyka accumulation. Tectonism along the convergent margin of Gondwana resulted in the formation of a foreland basin bounded by southern (Cape fold belt) and northern (Cargonian Highlands) uplands. Glaciers carved deep paleovalleys into the northern highlands that were later fi lled by glacigenic and post-glacial strata. Within this basin, a platform facies association composed of four degla-ciation sequences occurs. These sequences, which are hundreds of meters thick, consist of thick, massive, basal diamictite lithofacies that grade upward into stratifi ed litho-facies (stratifi ed diamictites, dropstone-bearing mudrocks, and rhythmites). Interpre-tations depict grounded ice advancing into the basin followed by gradual retreat of the ice front resulting in ice-proximal followed by ice-distal glaciomarine sedimentation. Sensitive high-resolution ion microprobe (SHRIMP) dates of juvenile zircons obtained from tuff beds indicate that the deglaciation cycles were 3.6–8.2 m.y. in duration. Such cycles were likely the result of tectonic development of the foreland basin. Paleocurrent and provenance studies indicate that Dwyka glaciation asynchronously emanated from multiple glacial centers in upland areas, and in Antarctica. Therefore, southern Africa was not covered by a single ice sheet, but instead, smaller ice sheets, ice caps, and alpine glaciers waxed and waned along basin margins during the late Paleozoic. Despite a long history of study, many questions concerning Dwyka glaciation remain.
... For instance, Milana and L6pez (1998) analysed the glacial to postglacial transition at the h'o Francia section, which is located in the central part of the PC Basin ( Fig. 4) and recognised several types of rhythmites ranging from typical varve-like couplets to others composed of gravelly sandstones and shales containing dropstones (Fig. 5E). Similar deposits have also been reported from other sections of the basin (L6pez Gamundi, 1991;Pazos, 2000b) and also from several Gondwanan basins including the Tepuel-Genoa (Gonzdlez Bonorino, 1992), ParanB (Gravenor et al., 1984) and GKK basins (Visser, 1994). ...
... In the GKK basin, the palaeogeographic reconstruction of Visser (1991) suggests an open marine realm to the southwest (Fig. 3) and a series of inland valleys, carved during glacial onset and filled during the retreat phase (Visser, 1994(Visser, ,1997. Glaciogenic facies are thicker in the southwest and thinner in the north-northeast (Visser, 1997). ...
Sedimentological, palaeontological and geological data from the glacial to postglacial transition in the late Paleozoic successions of the Paganzo-Calingasta Basin (PC) in southern South America and the Great Karoo-Kalahari Basin (GKK) in southern Africa are analysed, revised and reinterpreted. A brackish depositional setting is inferred for main areas previously considered to be nonmarine based upon ichnological interpretations. Three stratigraphic intervals have been defined based on changes in sedimentary facies and trace fossils association: The glacial interval (GI), early postglacial interval (EPI) and late postglacial interval (LPI). The GI and EPI contain a dominance of arthropod trackways, fish trails with and subordinate grazing and feeding traces. The EPI in the PC Basin comprises both nonmarine and brackish-marine ichnocoenoses without significant differences in ichnological composition. Trace fossils are preserved in underflow and turbidite beds of deltaic deposits. Opportunistic grazing traces constitute a post-event ichnocoenosis, while a pre-event ichnocoenosis is preserved at the base of turbidite beds. In the GKK Basin ichnofossils were documented in turbidite fans. The LPI in the GKK Basin contains the first evidence of shallow water deltaic infauna and subordinate grazing traces. Conversely, in the PC Basin the infauna is lacking.
... However, due to the textural similarities between some glacial and gravity flow deposits, a careful analysis must be carried out in glaciomarine environments in order to discriminate the origin of the mudstones. Such a discrimination is particularly challenging in pre-Quaternary glacial intervals (Visser 1983(Visser , 1994. Intraformational clasts of diamictitic material (till pellets of Ovenshine 1970, or till clots of Edwards 1978 are scarce, even at centimetric scale. ...
This study developed a novel, detailed sedimentological analysis for the complex interactions between rainout, iceberg raft�ing, tractional underfows, and settling of fnes along a glacially infuenced basin margin. The glaciomarine interval of the
El Imperial Formation (Pennsylvanian, Serpukhovian–Bashkirian) in the San Rafael basin comprises massive to stratifed
diamictites, interpreted as rainout tills, thinly bedded diamictites, associated with cohesive debris fows, and mudstones
containing ice-rafted debris (IRD), all capped by postglacial, transgressive, fne-grained sediments. The rhythmic inter�calation of IRD-bearing (dropstone mudstones) and IRD-free (mudstones) intervals likely indicates variations in debris
content within the ice margins, the on-and-of switching of ice streams, or dynamic oscillations of the ice terminus. The
glaciomarine deposits exhibit soft sediment deformation on both large (metric to decametric) and small (centimetric) scales.
This contribution refnes previous interpretations of the soft sediment deformation, discerning between loading and slope
triggered deformation. Large-scale deformation is characterized by coherent slump folds with low dispersion in the orien�tations of fold axial plane vergence and fold b-axes. Downslope-verging folds indicate a northward paleoslope, consistent
with paleofow indicators from fute casts found in sandstone turbidite beds. The diamictites afected by the large-scale soft
sediment deformation are interpreted as rainout tills with a variable degree of gravity remobilization. Their association with
thinly bedded diamictites and laminated mudstones with dropstones suggests that ice rafting played a signifcant role in the
deposition of this succession
... evidence of past glaciations; however, the diamictites can form by a variety of process in glacial, glacially influenced and non-glacial settings (Arnaud & Eyles, 2002;Delpomdor et al., 2017;Dietrich et al., 2019;Eyles & Januszczak, 2004, 2007Le Heron et al., 2017;Le Heron & Vandyk, 2019;Moxness et al., 2018;Soreghan et al., 2009;Vesely et al., 2018;Visser, 1994). Furthermore, diamictites often record tectonic events related to basin evolution and localised glaciation of uplifted margins. ...
The Cryogenian geological record of Siberia is scarce and ambiguous. Late Neoproterozoic strata of presumably glaciogenic origin of the Marnya Formation, Oselok Group cropping out along the Uda River in the Eastern Sayan Ranges, south‐western Siberia has received considerable attention due to the presence of at least three distinctive diamictite units. The lower diamictite unit (Karapchetui Member) is in subvertical contact (previously interpreted as a glacial valley) with stratigraphically older strata of the Tagul and Ipsit formations of the Karagas Group, and is represented by a wedge‐shaped unit of breccia that hosts numerous ellipsoidal sandstone bodies (previously thought to be boulders). The boulders are here reinterpreted as early diagenetic quartz and feldspar‐cemented sandstone concretions exhumed and redeposited from the Ipsit Formation when the latter was still uncemented and easily erodible. Tectonic compression, reverse faulting and localised continuous syndepositional uplift led to exhumation of the concretions, whereas subsequent extension, reactivation of the fault and ‘negative inversion’ of the basin produced accommodation space for redeposition of the exhumed concretions. In the process of redeposition, exfoliating concretions produced abundant debris that provided clasts for the breccia deposit. The Karapchetui diamictite, therefore, can serve as a sedimentary archive of late Neoproterozoic tectonic activity at the south‐western margin of the Siberian Craton.
... Although Nikolaev (1930) considered the Chivida Formation diamictites to be sediments of the continental moraine, after almost one hundred years of study of both ancient and modern glacial deposits, the Chivida Formation's set of sedimentary features is considered more typical for subaqueous glacial sediments. The thick monotonous successions of massive tills accumulate in quiet water under floating ice through fallout of suspension from a water column and ice rafted component, or subsequent reworking of these deposits in subaqueous debris flows (Eyles and Eyles, 1983;Fairchild and Hambrey, 1984;Gravenor et al., 1984;Hambrey and Glasser, 1978;Visser, 1983Visser, , 1994. The chaotic orientation of clasts, absence of sorting and signs of stratification, indicate prevalent rain-out sedimentation of the Chivida Formation's diamictites. ...
... These platform sequences are in turn truncated and overlapped by a regional cover of diamictites and interbedded black shales of the Dwyka Group that vary between 530 and 750 m in thickness (Linol et al., 2016b). This glacial sequence is Carboniferous in age and it defines the base of the Karoo Supergroup (Visser, 1994(Visser, , 1995(Visser, and 1997Schulz et al., 201 ). It is conformably overlain by 50 to 300 m thick black 8 shales (formerly "Upper Dwyka Shales", now named the Prince Albert Formation), which grade up into the 30 to 80 m organicrich shales of the Whitehill Formation ( Johnson et al., 2006;Chukwuma and Bordy, 2016;Schulz et al., 2016, 201 ). ...
The Karoo Basin of South Africa was explored for conventional oil and gas in the 1960s, and during which 19 boreholes were drilled by SOEKOR-the Southern Oil Exploration Corporation. Ten of these boreholes in the southern part of the basin were deep, c. 2.3 to 5.5 km in depth. Geochemical, petrophysical and petrographic studies on the recovered cores from these drill holes concluded that the lower Karoo sequences were possibly prospective for thermogenic gas across the Karoo region immediately north of the Cape Mountains and south of the main concentration of dolerite intrusions flanking the Karoo Escarpment. Since then, very limited analytical work has been undertaken on these cores to gain new data and to extend the concepts towards potential shale gas plays.
In the light of the recent interest in unconventional shale gas potential of the Karoo Basin, we re-examine this SOEKOR data and, together with new petrographic and geochemical analyses on 115 core samples from eight of the deep drill holes, we present new resource estimates of two possible reservoirs with recoverable shale gas of 10 to 50 Tcf (Source Rock 1) and 65 to 400 Tcf (Source Rock 2), respectively.
... A subaqueous mass flow origin for the primary diamicts can be ruled out considering the sheet-like geometry, presence of ice-rafted debris and parallel lamination, scarcity of soft-sediment deformation, no correlation between the bed thickness and the maximum clast size, and lack of associated facies unambiguously produced by sediment gravity flows (e.g., turbidites). However, redeposition of the thick-bedded diamictites by such flows, i.e., cohesive debris flows (Mulder and Alexander, 2001;Talling et al., 2012), is implied by their stacked nature (less than 2 m thick beds), sharp non-erosive basal contacts (probably as a result from hydroplaning of individual flows), lack of internal structures (except slumped beds), grading and preferred orientation of the clasts, formation of load casts, very poor sorting, presence of extraclasts derived from basement lithologies, locally abundant mud matrix and mudstone intraclasts, and absence of disturbance below lonestones (see López-Gamundí, 1991;Visser, 1994;Mulder and Alexander, 2001;Eyles and Januszczak, 2004;Arnaud and Eyles, 2006;Dobrzinski and Bahlburg, 2007;Ampaiwan et al., 2009;Henry et al., 2010Henry et al., , 2012Arnaud and Etienne, 2011;Couto et al., 2013;Isbell et al., 2016). In the same context, it is noteworthy that some of the above described textural characteristics, e.g., outsized clasts, galaxy/turbate structures and sediment aggregates (pellets), are known from deposits of subaqueous debris flows. ...
Glaciomarine deposits of late Hirnantian age in the western part of the Palaeozoic Balkan Terrane have persistent thickness (~ 7 m) and lateral uniformity in rock colour, bedding pattern, lithology, and sedimentary structures. Four lithofacies are distinguished from base to top: lonestone-bearing diamictites, interbedded structureless mudstones, crudely laminated diamictites, and finely laminated mudstones. The diamictites are clast-poor to clast-rich comprising muddy to sandy varieties. Their compositional maturity is evidenced by the very high amount of detrital quartz compared to the paucity of feldspar and unstable lithic grains. Other textural components include extraclasts derived from the local Ordovician basement, mudstone intraclasts, and sediment aggregates. Turbate structures, grain lineations, and soft sediment deformation of the matrix below larger grains are locally observed. Sedimentological analysis reveals that deposition occurred in an ice-intermediate to ice-distal, poorly agitated shelf environment by material supplied from meltwater buoyant plumes and rain-out from ice-rafted debris. Remobilization by mass-flow processes (cohesive debris flows and slumps) was an important mechanism particularly for the formation of massive diamictites. The glaciomarine deposits represent a typical deglaciation sequence reflecting retreat of the ice front (grounded or floating ice sheet), relative sea-level rise and gradually reduced sedimentation rate with increasing contribution from suspension fallout. This sequence was deposited on the non-glaciated shelf of the intracratonic North Gondwana platform along the southern margin of the Rheic Ocean. The Hirnantian strata of the Balkan Terrane can be correlated with similar glaciomarine deposits known from peri-Gondwana terranes elsewhere in Europe showing clear ‘Armorican affinity’. Several lines of evidence suggest that the provenance of siliciclastic material was associated mainly with sedimentary recycling of mature sands which had been deposited across North Gondwana in Cambrian and pre-glacial Ordovician times.
... Accordingly, the gradual transition from clast-poor diamictite to overlying sandstone and siltstone is in agreement with such interpretation (Evans and Pudsey, 2002). Variable clast concentrations in massive diamictites facies reflect a change in supply rate of the ice-rafted debris, and the absence of textural sorting in massive diamictites facies is probably due to their relatively rapid accumulation under quiet water conditions (Visser, 1994). ...
The Snowball Earth hypothesis suggests that the sea water had totally been kept frozen for millions of years in Neoproterozoic glaciation, followed by a rapid and catastrophic deglaciation resulting from the elevated concentration of atmospheric CO2. However, the sedimentary records are not consistent with the Snowball Earth hypothesis. The Nantuo Formation in Three Gorges area is composed of diamictites, sandstones and siltstones. The geochronology and the unconformity with underlying Liantuo Formation indicate that the Nantuo Formation in Three Gorges area may be the partial record (i.e., the final stage) of the Nantuo glaciation. Our studies on sedimentary successions of the Nantuo Formation convince the stepwise transition from the Cryogenia icehouse to the Ediacaran greenhouse, in which multiple glacier advanceretreat cycles rather than a catastrophic termination could be identified.
... The term "diamictite" is defined as "A comprehensive, nongenetic term proposed by Flint et al. (1960) for a nonsorted or poorly sorted, noncalcareous, terrigenous sedimentary rock that contains a wide range of particle sizes, such as rock with sand and/or larger particles in a muddy matrix" (Bates and Jackson, 1987). Until recently, diamictites have been regarded only as a lithification product of glacial till, and therefore have often been termed "tillites" (e.g., Eyles and Eyles, 1983;Visser, 1994;von Brunn, 1994). Impact cratering is now widely accepted to be of fundamental importance for the evolution of all planetary bodies with a solid surface (e.g., Silver and Schultz, 1982;Melosh, 1989;Grieve, 1991;Dressler et al., 1994;French, 1998;Montanari and Koeberl, 2000). ...
Diamictites are poorly sorted sediments characteristically carrying coarse-grained clasts in a fine-grained matrix. They have generally been considered of glaciogenic or glaciomarine origin. Recently, however, it has been suggested that some massive tillite/diamictite layers could represent impact breccias. An earlier petrographic study of rock and mineral clasts from Dwyka Group diamictites revealed no evidence for shock metamorphism, such as planar deformation features. Detailed geochemical studies of diamictite samples from the Archean Witwatersrand Supergroup and the Dwyka Group of the Mesozoic Karoo Supergroup from South Africa are reported. We studied the contents of the siderophile elements in these breccias, as elevated abundances of such elements, especially iridium, could be indicative for an impact origin. By use of γ-γ coincidence spectrometry and other trace element analysis, geochemical tracers of extraterrestrial components were sought. However, no enrichments of indicator elements for extraterrestrial components, compared with ordinary continental crust, were found. Thus, neither geochemical nor petrographic evidence supports an impact origin of the diamictites from the Dwyka Group and the Witwatersrand Supergroup in South Africa.
... The massive and weakly stratified diamictites were deposited by a combination of rain-out of mud, sand, and clasts from floating icebergs; rain-out of clay, silt, and sand from meltwater plumes emanating from the base of a tidewater glacier; and subaqueous debris flows (cf. Eyles et al., 1985;Dowdeswell et al., 1994;Visser, 1994;Woodworth-Lynas andDowdeswell, 1994, Powell andDomack, 2002). A lack of internal structures in diamictite is common to deposits of rain-out and debris flows. ...
The Wynyard Formation of Tasmania, Australia, provides the youngest evidence of grounded ice during the
Late Carboniferous–Early Permian in the Tasmania Basin of southeastern Gondwana during the late Paleozoic
ice age. Within the Wynyard Formation, four facies associations are recognized: 1) massive diamictite, 2)
stratified diamictite, 3) conglomerate and sandstone, and 4) deformed mudstone and fine sandstone. A detailed
facies analysis was performed in order to interpret the depositional processes and environments.
The massive diamictite facies association contains massive diamictite and sandstone and was deposited primarily
by iceberg rain-out in a subaqueous morainal bank setting. The stratified diamictite facies association
is composed of stratified diamictite, sandstone, and conglomerate, and was deposited by debris flows and iceberg
rain-out, and deformed by glacial pushing. The conglomerate and sandstone facies association is made
up of cross-stratified, channelized conglomerate and pebbly sandstone that were deposited by glacial outwash
on grounding-line fans. The deformed mudstone and fine sandstone facies association is composed of
pervasively deformed pebbly mudstone and fine-grained sandstone that exhibit volcano, flame, and dyke
structures. This facies association was deposited by a combination of suspension settling, iceberg rain-out,
and soft sediment deformation, in a quiet distal proglacial setting. The facies analysis supports the interpretation
that the Wynyard Formation was deposited by a wet-based, tidewater glacier or glaciers. The depositing
glacier(s) occupied a ~40 km-wide trough in northwest Tasmania. Based on the distribution of the
Wynyard Formation and the presence of glacial deposits in other eroded troughs in Tasmania, glaciation occurred
in the region within broadly eroded valleys. The Wynyard Formation and overlying mudstone of the
Inglis Formation provide a glacial–postglacial signature similar to those in South Africa, Antarctica, and eastern
Australia during the latest Carboniferous–Early Permian, therefore the climate warmed in southern
Gondwana following this glacial interval. The continued glaciation in eastern Australia was a result of regional
topography and oceanographic processes.
... The massive diamictite within the Ko He Formation is characteristic of a debris-flow deposit (Miall 1983;Eyles 1990;Visser 1994;Martin 1999). The origin of poorly-and well-stratified diamictites with tabular and lensoidal geometries is also generally ascribed to debris flows (Eyles 1990;Zheng et al. 1994;Eyles et al. 2001). ...
It has previously been proposed that the Sibumasu block of Southeast Asia, which contains glaciomarine deposits, became detached from the Gondwana margin during the Early Permian. A combination of facies analysis and the identification of dropstones and dump structures from a Lower Permian diamictite-bearing sequence at Phuket, Thailand, and adjacent islands suggests that the sediments originated as glaciomarine and debris-flow deposits. The Lower Permian diamictite-bearing sequence in the study area corresponds to the Ko Sire and Ko He Formations, both of which consist of three principal lithofacies: diamictite, sandstone, and fine-grained facies. The low-lying Ko Sire Formation is up to 400 m thick and is characterized by laminated mudstone; the presence of dropstones and dump structures associated with Cruziana ichnofacies indicates ice-rafted sedimentation in a glacially influenced offshore area. The Ko Sire Formation is overlain by a diamictite sequence of the Ko He Formation (up to 400 m thick). Poorly and well-stratified diamictites with tabular and lensoidal geometries, in combination with resedimentation textures, indicate that the diamictites within the Ko He Formation are debris-flow deposits. The similar lithology of clasts in the diamictites and dropstones possibly suggests that the debris-flow diamictite was presumably remobilized from pre-existing glacial deposits. The evidence of a glacially influenced offshore environment supports a previously proposed paleogeographic interpretation in which the Sibumasu block was most likely located at the Northwest Australian margin of Gondwana.
This study developed a novel, detailed sedimentological analysis for the complex interactions between rainout, iceberg rafting, tractional underflows, and settling of fines along a glacially influenced basin margin. The glaciomarine interval of the El Imperial Formation (Pennsylvanian, Serpukhovian–Bashkirian) in the San Rafael basin comprises massive to stratified diamictites, interpreted as rainout tills, thinly bedded diamictites, associated with cohesive debris flows, and mudstones containing ice-rafted debris (IRD), all capped by postglacial, transgressive, fine-grained sediments. The rhythmic intercalation of IRD-bearing (dropstone mudstones) and IRD-free (mudstones) intervals likely indicates variations in debris content within the ice margins, the on-and-off switching of ice streams, or dynamic oscillations of the ice terminus. The glaciomarine deposits exhibit soft sediment deformation on both large (metric to decametric) and small (centimetric) scales. This contribution refines previous interpretations of the soft sediment deformation, discerning between loading and slope triggered deformation. Large-scale deformation is characterized by coherent slump folds with low dispersion in the orientations of fold axial plane vergence and fold b-axes. Downslope-verging folds indicate a northward paleoslope, consistent with paleoflow indicators from flute casts found in sandstone turbidite beds. The diamictites affected by the large-scale soft sediment deformation are interpreted as rainout tills with a variable degree of gravity remobilization. Their association with thinly bedded diamictites and laminated mudstones with dropstones suggests that ice rafting played a significant role in the deposition of this succession.
The Devonian/Carboniferous Boundary (DCB) interval is associated with mass extinction, isotope excursions and a short glacial episode. This study investigates how boundary extinction and environmental change is expressed in the glacial high-palaeolatitudinal record of the Bolivian Altiplano (western Gondwana). A latest Devonian and early Carboniferous section has been investigated using sedimentology, palynology, total organic carbon and bulk δ ¹³ C organic . The Colpacucho Formation is a Late Devonian shelfal–marine siliciclastic sequence. It is overlain in the study area by a unit of coarse sandstones and sandy diamictites, interpreted as glaciomarine. This distinctive glaciomarine unit is at least 7 km wide and 60–120 m thick with a variably incisive basal contact (<100 m). It is of very latest Famennian age and is a stratigraphic equivalent of proven glacigenic deposits across central South America. The offshore marine Kasa Formation overlies the glacigenic unit above a basal flooding surface. The DCB is 12 m above this flooding surface on the last occurrence of Retispora lepidophyta and significant palynological assemblage changes. This includes the loss of the Umbellasphaeridium saharicum phytoplankton bioprovince, endemic to Gondwana. Marine and terrestrial palynological extinctions are synchronous with a 2 ‰ positive carbon isotope excursion interpreted to be reflective of changes in organic matter delivery and preservation during an interval of environmental stress. These results inform wider debates on global environmental change and mass extinction at the DCB.
The currently favored hypothesis for Late Paleozoic Ice Age glaciations is that multiple ice centers were distributed across Gondwana and that these ice centers grew and shank asynchronously. Recent work has suggested that the Transantarctic Basin has glaciogenic deposits and erosional features from two different ice centers, one centered on the Antarctic Craton and another located over Marie Byrd Land. To work towards an understanding of LPIA glaciation that can be tied to global trends, these successions must be understood on a local level before they can be correlated to basinal, regional, or global patterns. This study evaluates the sedimentology, stratigraphy, and flow directions of the glaciogenic, Asselian–Sakmarian (Early Permian) Pagoda Formation from four localities in the Shackleton Glacier region of the Transantarctic Basin to characterize Late Paleozoic Ice Age glaciation in a South Polar, basin-marginal setting. These analyses show that the massive, sandy, clast-poor diamictites of the Pagoda Fm were deposited in a basin-marginal subaqueous setting through a variety of glaciogenic and glacially influenced mechanisms in a depositional environment with depths below normal wave base. Current-transported sands and stratified diamictites that occur at the top of the Pagoda Fm were deposited as part of grounding-line fan systems. Up to at least 100 m of topographic relief on the erosional surface underlying the Pagoda Fm strongly influenced the thickness and transport directions in the Pagoda Fm. Uniform subglacial striae orientations across 100 m of paleotopographic relief suggest that the glacier was significantly thick to “overtop” the paleotopography in the Shackleton Glacier region. This pattern suggests that the glacier was likely not alpine, but rather an ice cap or ice sheet. The greater part of the Pagoda Fm in the Shackleton Glacier region was deposited during a single retreat phase. This retreat phase is represented by a single glacial depositional sequence that is characteristic of a glacier with a temperate or mild subpolar thermal regime and significant meltwater discharge. The position of the glacier margin likely experienced minor fluctuations (readvances) during this retreat. Though the sediment in the Shackleton Glacier region was deposited during a single glacier retreat phase, evidence from this study does not preclude earlier or later glacier advance–retreat cycles preserved elsewhere in the basin. Ice flow directions indicate that the glacier responsible for this sedimentation was likely flowing off of an upland on the side of the Transantarctic Basin closer to the Panthalassan–Gondwanide margin (Marie Byrd Land), which supports the hypothesis that two different ice centers contributed glaciogenic sediments to the Transantarctic Basin. Together, these observations and interpretations provide a detailed local description of Asselian–Sakmarian glaciation in a South Polar setting that can be used to understand larger-scale patterns of regional and global climate change during the Late Paleozoic Ice Age.
Along the easternmost edge of the Karoo–Kalahari Basin (KKB) of Botswana, the Toutswemogala Hill succession exposes a 30–50-m-thick suite of siliciclastic deposits interpreted by some as glaciogenic in origin tied to the Permo-Carboniferous Late Paleozoic Ice Age (LPIA). Six facies associations (FA) were recognized in this succession, resting unconformably on a highly uneven Archean gneissic basement, and consisting from base to top of: 1) clast-supported breccia made up of angular cobbles and boulders ubiquitously derived from the underlying basement, 2) well-bedded siltstones sealing or locally interdigitated with the underlying breccia, and bearing abundant remnants of Glossopteris sp. leaves, 3) a chaotic to faintly laminated matrix-supported diamictite bearing angular and subrounded clasts and tree logs attributed to the genus Megaporoxylon, 4) cross-bedded conglomerate bearing well-rounded quartz and clasts, 5) planar-laminated to ripple-laminated, poorly sorted, muddy sandstones showcasing dispersed mud chips that grade upward into 6) poorly sorted, cross-bedded coarse-grained sandstones displaying convolute beds and abundant imprints of unidentifiable tree logs.
No evidence of glaciogenic processes have been found in this succession, in the form of either pavement or clasts striations. The breccia and diamictite are interpreted as scree and mass-flow deposits, respectively. Along with the age of the deposits, inferred from the plant debris (upper Carboniferous to lower Permian), the stratigraphic position of this sedimentary succession resting on the Archean basement suggests that it corresponds to the Dukwi Formation, a stratigraphic equivalent of the Dwyka Group in the Main Karoo Basin. This would explain the resemblance of the facies to those recovered at the base of the central Kalahari–Karoo Basin and in the neighboring Tuli, Ellisras, and Tshipise basins. The absence of diagnostic criteria for glacial processes in the studied succession raises the question of the extent, in both time and space, of the LPIA-related ice masses over southern Africa and particularly in southeastern Botswana. It is suggested here that during this glacial epoch, spatially restricted ice masses were confined in bedrock valleys (valley glaciers) in an uplifted setting otherwise characterized by non-glaciogenic processes, further strengthening the scenario of fragmented ice masses over southern Gondwana.
The Agua de Jagüel Formation near Uspallata, Mendoza Province, Argentina, was deposited within a
palaeovalley along the southeastern margin of the Calingasta–Uspallata Basin adjacent to the Protoprecordillera.
The basal glacigenic sequence of the formation was deposited in the Serpukhovian–Early
Bashkirian, near the beginning of the late Palaeozoic ice age. The sequence is composed of diamictite,
conglomerate, sandstone, and mudrock and records four depositional stages within a palaeofjord: 1. morainal
bank deposition by a wet-based tidewater glacier, 2. glacial retreat succession where ice retreated up the
fjord out of the immediate area, allowing iceberg deposition of dropstones and dump deposits, 3. continued
glacial retreat with ice receding onto land, allowing a shoreface to develop within the palaeovalley, and
4. transgression across the shoreface and resumption of iceberg deposition. The thickness and facies of this
succession are similar to deposits within modern Alaskan fjords housing temperate tidewater glaciers.
Comparison of the Agua de Jagüel Formation with Alaskan fjords indicates that sequence stratigraphy used
for low latitude deposits must be applied with caution, as glacimarine ice retreat within a fjord produces
strata surfaces that are easily misidentified as flooding surfaces resulting from changes in water depth.
Likewise, the transgression during stage 4 is significant as it indicates a rise in sea level that occurred during
glaciation of the Protoprecordillera, thus suggesting that the transgression between stages 3 and 4 was not
driven by input of glacial meltwater. Additionally, the sediment geochemistry suggests that the bottom
waters of the palaeovalley were anoxic, which may help explain the absence of bioturbation in the sequence
as well as in other ancient glacimarine palaeovalleys. The deglaciation succession and transgression are also
recorded in the nearby Hoyada Verde and Tramojo Formations, so it is proposed that all three formations
record one glacial event in the Protoprecordillera. Overlying fluvial and shallow marine strata in the Agua de
Jagüel, Hoyada Verde, and Tramojo Formations show no indication of continued glaciation in the
Protoprecordillera following stage 4 and equivalent strata. The deglaciation succession of the Agua de
Jagüel Formation affirms the emerging concept that the late Palaeozoic ice age was characterized by alpine
glaciers, ice caps, and small ice sheets that were not massive enough to have driven eustatic fluctuations of
100 m+ as previously understood, and that ice never covered westernmost Gondwana during later LPIA
events.
The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice-rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles.
The massive diamictites formed as aprons and fans in front of the ice-grounding line, whereas the stratified diamictites represent more distal debris-flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes.
Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter-term fluctuation of the ice margin is present within the major advance-retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea-level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono-eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer-term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.
The unique and evolving nature of the Precambrian geological environment in many ways was responsible for significant differences between Precambrian clastic sedimentary deposits and their Phanerozoic-modern equivalents. Some form of plate tectonics, with rapid microplate collisions and concomitant volcanic activity, is inferred to have led to the formation of greenstone belts. Explosive volcanism promoted common gravity-flow deposits within terrestrial greenstone settings, with braided alluvial, wave/storm-related and tidal coastline sediments also being preserved. Late Archaean accretion of greenstone terranes led to emergence of proto-cratons, where cratonic and rift sedimentary assemblages developed, and these became widespread in the Proterozoic as cratonic plates stabilised. Carbonate deposition was restricted by the paucity of stable Archaean terranes.
Ocean Drilling Program Leg 178 (February–April 1998) drilled two sites (Sites 1097 and 1103) on the outer Antarctic Peninsula Pacific continental shelf. Recovered strata are no older than late Miocene or early Pliocene (<4.6 Ma). Recovery at shallow depths in loosely consolidated and iceberg-turbated bouldery sediment was poor but improved with increasing depth and consolidation to allow description of lithofacies and biofacies and interpretation of depositional environment.Site 1097 lies on the outer shelf within Marguerite Trough which is a major outlet for ice expanding seaward from the Antarctic Peninsula and reached a maximum depth drilled of 436.6 m below the sea floor (mbsf). Seismic stratigraphic data show flat-lying upper strata resting on strata that dip gently seaward. Uppermost strata, to a depth of 150 mbsf, were poorly recovered, but data suggest they consist of diamictites containing reworked and abraded marine microfauna. This interval is interpreted as having been deposited largely as till produced by subglacial cannibalization of marine sediments (deformation till) recording ice sheet expansion across the shelf. Underlying gently dipping strata show massive, stratified and graded diamictite facies with common bioturbation and slump stuctures that are interbedded with laminated and massive mudstones with dropstones. The succession contains a well-preserved in situ marine microfauna typical of open marine and proglacial marine environments. The lower gently dipping succession at Site 1097 is interpreted as a complex of sediment gravity flows formed of poorly sorted glacial debris.Site 1103 was drilled in that part of the continental margin that shows uppermost flat-lying continental shelf topsets overlying steeper dipping slope foresets seaward of a structural mid-shelf high. Drilling reached a depth of 363 mbsf with good recovery in steeply dipping continental slope foreset strata. Foreset strata are dominated by massive and chaotically stratified diamictites interbedded with massive and graded sandstones and mudstones. The sedimentary record and seismic stratigraphy is consistent with deposition on a continental slope from debris flows and turbidity currents released from a glacial source. Data from Sites 1097 and 1103 suggest the importance of aggradation of the Antarctic Peninsula continental shelf by till deposition and progradation of the slope by mass flow. This may provide a model for the interpretation of Palaeozoic and Proterozoic glacial successions that accumulated on glacially influenced continental margins.
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