Vertical Intercalation of Matrix-Rich and Matrix-Poor Sandstones - A Reflection of a Compensationally-Stacked Lateral Facies Change in a Deep-Marine Channel-Margin Succession, Isaac Formation, Windermere Supergroup, British Columbia.
Matrix-rich sandstones and associated matrix-poor sandstones are commonly observed in the deep-marine rock-record, yet their depositional mechanisms have been contested due to poorly documented lateral and vertical changes. At the Castle Creek study area, a remarkably well exposed, recently de-glaciated, vegetation free, 30m thick by 350m wide section containing both matrix-rich and matrix-poor strata was measured bed-by-bed to document changes in facies and stratal assemblages. Based on these data three distinctive stratal assemblages are observed; (SA1) matrix-poor, traction structured turbidities (Tbc, Tb, Tc); beds are several cm to dm thick and stack to form units 2-6m thick; (SA2) planar-based, structureless, coarse-tail graded, muddy sandstone/claystone (~30-70% matrix), with common mudstone chips; stacked several cm-thick beds that form units 0.5-2m thick. (SA3) matrix-poor sandstone (<10% matrix) overlain sharply by 2-3 muddy sandstone/claystone beds; dm to m-thick bedsets that stack to form units 1.5-4.5m thick. Here, like in previous work in the Windermere, individual matrix-rich sandstones undergo a systematic lateral facies change from matrix-poor sandstone (SA3) to muddy/claystone (SA2) – a transition interpreted to be related to particle settling in a negligibly-shearing sediment suspension outboard of an area of intense seabed erosion and mud ingestion. Stratigraphically upwards, SA2 and SA3 form a succession of sharply-bound intercalated units, which then implies the abrupt superposition of laterally adjacent stratal assemblages – specifically, the abrupt compensational superposition of different parts of the lateral facies transect, suggesting the abrupt lateral displacement of the axis of seabed scour and deposition. Significantly then, is the abrupt intercalation of thick SA1 units with their low matrix content and ubiquitous presence of traction-transport structures. Also, the grain size of the sand-size fraction varies little between units, suggesting that these changes are related to the abrupt intercalation of two end-member styles of deposition, specifically, particle settling and traction transport. These systemic changes, although currently poorly understood, must be related to differences in the magnitude of fluid shear within the advecting sediment dispersion and/or spatial/temporal changes in the local availability of an erodible muddy substrate that profoundly changed the rheology and density structure of the suspension.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017