Alignment of fluvio-tidal point bars in the middle McMurray Formation: implications for structural architecture of the Lower Cretaceous Athabasca Oil Sands Deposit, northern Alberta

The northern Athabasca Oil Sands Deposit accumulated on sub-Cretaceous structure partially configured by multistage pre-Cretaceous salt dissolutions in Prairie Evaporite (Middle Devonian) substrate that continued concurrent with deposition of McMurray Formation (Aptian) strata. Dissolution fronts only 250 m below advanced along NW- and NE-oriented fracture–fault lineaments that coalesced into larger salt removal areas. This structural grain was transmitted to the overlying dissected Upper Devonian karst topography draped by lower McMurray braided rivers along a lattice-like channel network. The dominant NW structural grain continued during middle McMurray deposition, with fluvial-estuarine point bars aligned along subparallel tidal channels. Regional salt removal fronts concurrent with middle McMurray deposition migrated north of the Bitumount Trough, resulting in the 200 km2 central collapse. The northern Athabasca Deposit area was configured as a funnel-shaped lower estuary structure consisting of aligned Upper Devonian – lower McMurray fault block terraces that stepped down northward into the central collapse. Sinuous river channels of the upper estuary, constrained along stable substrate of the main paleovalley, flowed northward onto the unstable floor of this funnel-form lower estuary. The main paleovalley fairway branched into multiple tens of kilometres long subparallel fluvio-estuarine tidal channels aligned parallel to the NW structural grain. Sand transport fairways cascaded over the step-down terraces and permitted aggradations of overlying fluvio-tidal point bars to accumulate into giant commercially attractive sand complexes. The internal architecture of these tens of metres thick sand deposits included deposit-wide erosion surfaces resulting from cycles of collapse–subsidence, stabilized substrate and erosion, and renewed subsidence and aggradation.

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THE SEDIMENTOLOGY AND ICHNOLOGY OF ESTUARINE POINT BARS IN THE McMURRAY FORMATION OF THE ATHABASCA OIL SANDS DEPOSIT, NORTH- EASTERN ALBERTA, CANADA

Applications of Ichnology to Petroleum Exploration ◽

10.2110/cor.92.01.0401 ◽

1992 ◽

pp. 401-421 ◽

Cited By ~ 11

Author(s):

Michael J. Ranger ◽

S. George Pemberton

Keyword(s):

Oil Sands ◽

Athabasca Oil Sands ◽

Mcmurray Formation ◽

North Eastern ◽

Point Bars

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Syndepositional architecture of the northern Athabasca Oil Sands Deposit, northeastern Alberta

Canadian Journal of Earth Sciences ◽

10.1139/cjes-2014-0021 ◽

2015 ◽

Vol 52 (1) ◽

pp. 21-50 ◽

Cited By ~ 17

Author(s):

Paul L. Broughton

Keyword(s):

Oil Sands ◽

Linear Chain ◽

Upper Devonian ◽

Karst Collapse ◽

Athabasca Oil Sands ◽

Northern Margin ◽

Sea Level Fluctuations ◽

Salt Dissolution ◽

Half Graben ◽

Spatio Temporal

Salt dissolution collapse-subsidence is proposed as the dominant tectono-stratigraphic control on the deposition of major sand trends across the northern Athabasca Oil Sands Deposit. Salt removal along linear dissolution trends 200 m below in the Prairie Evaporite (Middle Devonian) halite beds resulted in the collapse of the overlying Upper Devonian strata. The collapse-induced differential subsidence of the fault blocks formed the floor underlying the McMurray deposits in the 50 km long V-shaped Bitumount Trough extending across the northern area of the Athabasca Oil Sands Deposit. The lower and middle-upper McMurray sand trends filled the accommodation created by collapses of a linear chain of Upper Devonian fault blocks along the northern margin of the western Trough. A pair of tens-of-metres thick and 20–30 km long sand trends developed parallel in overlying accumulations of the lower and middle-upper McMurray Formation (Aptian). This half-graben tilted northward as the dissolution trend in the underlying Prairie Evaporite salt scarp widened, and the scarp margin was deeply embayed. Salt dissolution-induced structures were the principal control that located the large sand complexes exploited by bitumen mining projects. Earlier models of McMurray architecture interpreted the underlying karst collapse to have been largely pre-Cretaceous. This new architectural model reinterprets the spatio-temporal balance between erosion at the pre-Cretaceous surface and within the buried salt beds. Extensive salt removal resulted in collapse of the underlying hypogene karst during the late Aptian age. This resulted in the over-thickened multi-kilometres long McMurray sand trends. The underlying karst collapse resulted in unstable deposition surfaces along the sub-Cretaceous trough floors. This tectono-stratigraphic architecture, called the syndepositional model in this study, is proposed as an alternative to two other models, one of which proposes that deeply incised channel valleys and fills resulted from multiple significant sea-level fluctuations, while the other proposes that stacked parasequences accumulated along overlying shallow channels that meandered across a stable fluvio-estuarine coast.

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