Grand Rapids Formation, North-Central Alberta: An Example of Nearshore Sedimentation in a High Energy, Shallow, Inland Sea: ABSTRACT

Abstract

End_Page 589------------------------------The Lower Cretaceous Grand Rapids Formation in the Wabasca Oil Sand Deposit of north-central Alberta contains approximately 10.5 billion cu m (66 × 109 bbl) of viscous, low gravity bitumen in the subsurface. It represents a progradational clastic sequence building northwest into the Boreal Sea during early Albian time. The formation measures up to 90 m thick and consists of three progradational sandstone units (C, B, and A members in ascending order), separated by two marine shale/siltstone units, indicative of marine transgression. Sequences of lithologies, sandstone geometry, and sedimentary structures indicate that each sandstone member represents a variety of depositional environments. Each shows a shallowing-upward trend from nearshore marine below effective wave base to lower and upper shoreface facies. Tidal inlet, beach, and possibly coastal dune environments are found in the upper parts of the A and B sandstones. Variations between localities indicate that upper shoreface to ?dune environments laterally grade into lagoonal and/or terrestrial environments. Coals, which locally cap the A and B sandstones, indicate back-barrier lagoon or interchannel marsh deposits. Local chert pebble beds found in the upper shale/siltstone sequence and in the basal part of the A sand are interpreted as possible major storm deposits. On a regional scale, the Grand Rapids Formation of the Wabasca area appears to represent a barrier island coastal complex possibly adjacent to deltaic or delta plain sequences. End_of_Article - Last_Page 590------------