Glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples

Abstract Three variables control the type of glacial debris source and where and when debris is introduced to an ocean: (1) type of glacial ice source (ice sheet or valley glacier); (2) condition of a grounded glacier (melting/freezing or frozen base and cold or warm internal ice); and (3) type of glacier front (ice shelf or tidewater). Using these variables in different combinations eight glacimarine regimes are postulated to exist. By integrating information from Holocene sediment and Pleistocene glacimarine sequences with the eight inductive regimes, lithofacies relationships are predicted and modelled. The models show a complexity of glacimarine lithofacies. The importance of recognizing this complexity lies in realistic interpretations of ancient sequences. Ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation. Alternatively, in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes. Ice-proximal sediment is distinctive, especially near subglacial meltwater stream effluxes. Ice shelves and tidewater glaciers may produce similar ice-proximal lithofacies (e.g. morainal bank associations and laminites) in certain regimes. However, details of lithofacies may aid in distinguishing ice shelf from tidewater regimes. Ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high-level englacial debris can be distinguished. Ice shelf zone sediment out from the grounding line has a basal debris source. High-level englacial debris is deposited in more distal ice shelf zones resulting in sediment by-pass. In corresponding positions of a tidewater glacier regime, iceberg rafted debris comprises mixed basal and high-level englacial debris. Deeper water zones receive large volumes of sediment when a melting/freezing-base grounding line is at the edge of a continental shelf. When the grounding line retreats sediment supply may be drastically reduced because of a back-sloping continental shelf and trapping by morainal banks. Cool water bioclastic carbonates may accumulate once a grounding line has retreated. Although ice shelf iceberg zones are more extensive than those in tidewater regimes, ice shelf icebergs have low debris content and resulting ice-rafted debris may be scarce. The smallest volume of sediment is produced by a frozen-base ice sheet ending as a tidewater front, and the largest volume by a melting/freezing-base valley, tidewater glacier. Glacimarine sediment has a relatively high preservation potential near continental interiors or margins. However, at continental margins mass flow (olistostrome) and tectonic (melange) deposits may be preserved with glacimarine diamictites. These deposits may be difficult to distinguish from each other, especially if metamorphosed such as at a convergent margin. The models presented do not fully represent the glacimarine environment, because many other variables are omitted. However, the principal concepts of generating glacimarine lithofacies associations are considered. Continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models.