Sedimentology, Ichnology, and Depositional History of the Contact Between the Triassic Montney and Doig Formations in West-Central Alberta, Canada – Presence of an Unconformity Bound Sand Wedge

LaMothe, Jon T.¹, M.K. Gingras¹, S.G. Pemberton¹ (1) University of Alberta, Edmonton, AB

In most locations the boundary between the lower and middle Triassic Montney and Doig formations of West-Central Alberta is represented by fine-grained shoreface sands overlying phosphatic lag deposits within organic rich shale. This contact represents a submarine erosional event followed by a period of non-deposition. In contrast, around the Gordondale and Pouce Coupe fields this contact contains three major submarine erosive surfaces (E1, E2, E3) marked by phosphate lag deposits. These significant stratigraphic markers create two unconformity bound wedges informally named for this project the Moig Siltstone Unit and the Moig Sandstone Unit in ascending order.

Detailed analysis reveals that the lower unconformity bound wedge (E1-E2) contains sandy silts relating to deeper water facies, while the overlying sandy unconformity bound wedge (E2-E3) contains heavily bioturbated offshore to delta front facies. Sedimentary structures and ichnology help with interpretation of the Moig Siltstone Unit while only ichnology can be used on the Moig Sandstone Unit due to the high abundance of trace makers eradicating any previous structures. Mapping of the Moig Sandstone Unit shows numerous semicircular sand lobes. This is due to original depositional characteristics being planed off by the overlying erosive submarine event (E3), which was followed by a period of non-deposition creating phosphates. Since mapping of this unit shows no paleo-depositional history, core analysis is the main method of arriving at a depositional model. Through this analysis there is the recognition of offshore to delta front traces. This study concludes that a relatively shallow marine setting was present for the upper portion of the Moig Sandstone Unit when compared to other shallow marine ichnology models.