The Wilrich Member, Alberta deep basin: An example of a topsetdominated delta deposited into a low-accommodation shallow sea.

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1 The Wilrich Member, Alberta deep basin: An example of a topsetdominated delta deposited into a low-accommodation shallow sea. Kerrie Bann & Michael Fawcett Ichnofacies Analysis Inc. & Tourmaline Oil Corp. The sandstones of the Wilrich Member have previously been described as prograding marine shorelines. This viewpoint is largely derived from early studies of the upper Spirit River Falher units, which are often conglomeratic, and which show clear east-west shoreline trends in the northern deep basin. The sandstone trends in the Wilrich Member are not as distinct, and they are rarely conglomeratic. Although commonly mapped as northeast-southwest trending shorelines, the trace fossil suites and primary sedimentary features seen in the core clearly reflect deltaic conditions. Wilrich core was examined from Township 46 in the south to Township 62 in the north, a distance of over 200 kilometers. The core study indicates that the Wilrich interval is a north to northwest prograding, complex delta system. The reservoir units are deltaic shorelines, mouthbars and distributary channels. The facies over the entire distance were deltaic in nature, and the trace fossil assemblages reflect open marine associations that display a variety of fluctuating physicochemical stresses such as profound: salinity changes, hypopycnal-flow-induced water turbidity, distributary flood discharges with accompanying phytodetrital (comminuted plant debris) pulses, hyperpycnal-flow-induced sediment gravity flows, and fluid-mud deposition. All of these conditions conspire to produce depauperate ichnological assemblages. The degree of river, wave and storm influence differs between individual cores. The majority of the defining trace fossil suites and sedimentary features are observed in the prodeltaic and distal deltaic facies. Prodeltaic units display trace fossil suites that are sporadically distributed, reduced in size and diversity as compared to open marine counterparts and almost exclusively composed of deposit-feeding behaviours and opportunistic facies crossing elements. Typical suites contain Phycosiphon, Planolites, Palaeophycus tubularis, Cosmorhaphe, Zoophycos, Thalassinoides, Chondrites, Asterosoma, Teichichnus and complex Rhizocorallium (Figure 1). Not all of these ichnogenera are present in every prodeltaic unit. Most are dominated by two or three ichnospecies. Distal delta front deposits also show sporadic distribution of bioturbation and suites that are almost exclusively composed of deposit-feeding behaviours and opportunistic facies crossing elements. Typical ichnogenera include large Macaronichnus isp., Rosselia socialis, large complex Rhizocorallium, Diplocraterion habichi, Planolites, Thalassinoides and Palaeophycus tubularis. These types of trace fossil suites that show persistent reductions in bioturbation intensity and impoverishment in ichnological 1

2 diversity, compared to those of nondeltaic shorelines are described from a multitude of deltaic deposits in Canada, the United States, Australia, the South China Sea and offshore Norway (MacEachern et.al., 2005). The persistence of deltaic facies across this large area was surprising. The Lexicon of Canadian Stratigraphy indicates marine facies to the north around Township 78-80, but at present the eastern, southern, and western limits of the deltaic facies are unknown. A polygon taken from Township 70 south, encompassing the wells where core was examined, covers an area of 65,000 square kilometers. The large aerial expanse indicates a significant drainage basin, where the sediments were discharged into a low accommodation continental basin. The low accommodation space and absence of a shelf margin modifies the delta stratigraphy, producing a topset-dominated delta (Edmonds et al, 2011), see also Fielding et al, 2006 for modern examples of deposition in low accommodation settings. Recent work on the Aptian (lower Mannville) McMurray Formation indicates that it is the product of a continental scale drainage system (Benyon et al, 2016; Blum and Jenning, 2016; Leier and Leckie 2016). The lower Albian Wilrich member sits above the McMurray Formation, at the base of the upper Mannville. It is a reasonable speculation that the Wilrich deltaic sands were deposited from the same continental drainage system that supplied the McMurray Formation. 2

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