Carolyn H. Eyles

Models of glaciomarine sedimentation and their application to the interpretation of ancient glacial sequences

Abstract This paper argues that glaciomarine environments can be regarded as special, glacially-influenced types of continental margin environments (e.g. continental shelf, slope, rise and basin plain). Knowledge of the stratigraphic architecture and typical sedimentary sequences of non-glacial margins is becoming well-known but remains limited for those that have been glacially modified. The principal influences on sedimentation in these environments relate to the glacial sediment input (controlled by relief of basin margin, glacier thermal regime and ice flow dynamics) and depositional environments (influence of traction currents, substrate relief and proximity to nearby ice margins). Typical ranges of sedimentation rates can be established for glacially-influenced continental margin environments and these may provide a framework for ancient sequences. Starvation of sediment supply to glacially-influenced continental margins is common. The nature of sub ice shelf sedimentation, a model that has been applied to many ancient glacial sequences is critically reviewed; the significance of such sedimentation in the rock record has probably been exaggerated because of oversimplistic interpretations of diamictite sequences. Existing process models of glaciomarine sedimentation derived from study of modern environments are sometimes difficult to employ in investigation of ancient sedimentary sequences because simple lithofacies criteria and typical vertical profiles are not available to aid in interpretation. In addition compositional data emphasized by many workers for distinguishing glaciomarine from continental glacial diamict(ite)s frequently fingerprint sediment source and not mode of deposition. The importance of facies analysis methods for isolating depositional environments is illustrated by three examples of ancient glaciomarine sequences. These are the Early Proterozoic Gowganda Formation (∼2.3 Ga) of northern Ontario, Canada; the Late Proterozoic Port Askaig Formation (∼670 Ma) of Scotland and Ireland, and the Late Cenozoic Yakataga Formation (∼20 Ma to recent) of the Gulf of Alaska. These examples illustrate the significance of detailed genetic studies of ancient glacial rocks in the interpretation of palaeogeographic and tectonic settings. Diamictite units in ancient glaciomarine sequences cannot be easily interpreted in terms of climatic or ice advance/retreat cycles, because of the varied controls on diamict accumulation and diamictite preservation in marine basins.

Sedimentation in a large lake: A reinterpretation of the late Pleistocene stratigraphy at Scarborough Bluffs, Ontario, Canada

A lithofacies code developed for diamicts emphasizes facies variability in the classic upper Pleistocene glaciogenic sequence at Scarborough Bluffs, Ontario, Canada. Lithological breaks along the bluffs are traditionally assigned to incursions of grounded ice margins and to interstadial lakes formed during ice retreat, during the Early and Middle Wisconsin. Detailed sedimentological logging through three diamict units (previously formalized as Sunnybrook, Seminary, and Meadowcliffe Tills) and intervening sandy lithofacies shows absence of glaciotectonic structures and diamicts associated with grounded glacier ice, traction-current activity during diamict accumulation, postdepositional resedimentation of diamicts into topographic lows accompanied by turbidite activity, a subaqueous deltaic origin for intervening sandy lithofacies, and loaded, transitional, and interbedded contacts between sand and diamict. The Scarborough Bluffs sequence may be the preserved bottom stratigraphy of a large lake. The bottom stratigraphy results from repeated basinward progradation of deltaic sandy lithofacies over glaciolacustrine diamicts deposited below floating ice, whether ice shelf, ramp, bergs, or lake ice. A facies model is presented for glaciolacustrine diamict deposition on the floors of enlarged Pleistocene lakes trapping substantial volumes of fine-grained suspended sediment.

Glaciomarine sediments of the Isle of Man as a key to late Pleistocene stratigraphic investigations in the Irish Sea Basin

The northern part of the Isle of Man in the central Irish Sea Basin is identified as a raised marine foreland containing the largest subaerial exposure of last glaciation marine and glaciomarine sediments yet recognized on the British continental shelf. A large push ridge that crosses the foreland identifies a major ice limit. The ridge divides the foreland stratigraphy into a coarsening-upward marine sequence of pelagic muds with dropstones, sand, and gravel to the south from offlapping stratified and massive sandy diamict assemblages overlying glaciotectonized marine sediments to the north. Coarse-grained diamicts are argued to form part of a push-ridge-subaqueous-outwash depositional system deposited when processes of suspension deposition, density underflow, ice rafting, highly variable traction-current activity, and sediment gravity flow were operative adjacent to a grounded marine ice margin. The identification of large offshore submarine banks aligned parallel to the Bride moraine suggests that construction of the push-ridge-subaqueous-outwash depositional system may have been repeated during ice recession. This and published descriptions of other “Irish Sea tills” indicate that analogous structures and sediments are present offshore and around the Irish Sea Basin.

Glacially- and tidally-influenced shallow marine sedimentation of the late Precambrian Port Askaig Formation, Scotland

The late Precambrian Port Askaig Formation forms part of the thick succession of metasediments and volcanics of the Dalradian Supergroup of northwest Britain. The glacianegnic Port Askaig Formation is well exposed on the Garvellach Islands in western Scotland and consists of a series of thick and regionally extensive diamictites interbedded with sandstones, siltstones, dolomites and minor conglomerates. Diamictite facies are similar to glaciomarine diamicts described from the Late Cenozoic Yakataga Formation in the Gulf of Alaska, formed by settling of fine-grained suspended sediment and ice-rafting in relatively shallow water (< 250 m). Two particularly distinctive diamictite units, the Great Breccia and the Disrupted Beds are interpreted as the product of downslope resedimentation events; it is significant that the Port Askaig Formation was deposited during a period of increasing tectonic activity in the Dalradian Basin. Thick and extensive sand sequences interbedded with the diamictite facies are interpreted as tidally influenced shallow marine shelf deposits; sands may have been derived from deltaic sources on the basin margin and transported into the basin during storms. The entire Port Askaig Formation appears to represent an overall shallowing upwards sequence punctuated by smaller scale transgressive (diamictite) and regressive (sandstone) events. Interpretation of the Port Askaig Formation as a succession of glacially and tidally-influenced shelf sediments has significant implications for the reconstruction of palaeoclimatic histories and regional palaeogeographies during late Precambrian time.

Glaciomarine model for upper Precambrian diamictites of the Port Askaig Formation, Scotland

Descriptive techniques (lithofacies codes) and vertical profile analysis emphasizing lithofacies contacts and geometry allow objective characterization and comparison of diamict(ite) sequences irrespective of geologic age. Upper Precambrian diamictites (?650−600 m.y.) of the Middle Dalradian Port Askaig Formation in Scotland are traditionally interpreted as products of repeated ice-sheet advances over an emergent shallow marine shelf and deposition by in situ basal melt-out. The Dalradian sequences are, however, fundamentally different from modern and Pleistocene diamicts deposited by either lodgment or basal melt-out and fail to show evidence of tractional processes associated with basal ice movement. Scottish diamictite sequences are similar to assemblages of massive and stratified diamict lithofacies, of late Pleistocene age, deposited subaqueously below floating ice and recently described from the Lake Ontario Basin of North America. A glaciomarine depositional model may have implications for the origin and paleogeographical setting of other upper Precambrian diamictites of similar age and stratigraphic context in East Greenland, Spitsbergen, and Scandinavia.