Graded or Out-of-Grade Stratigraphic Character of Basin-Floor Deposits -- Thoughts from the Neoproterozoic Windermere Supergroup

Arnott, R.W.C. and Terlaky, V.
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Strata of the Neoproterozoic Windermere Supergroup (WSG) crop out over 3000 km from northern Mexico to the Alaska-Yukon border, and represent continental to deep-marine sedimentation along the newly formed passive margin of Laurentia (ancentral North America). Deep-marine siliciclastic rocks of that system crop out superbly in the southern Canadian Cordillera where they form a sedimentary pile about 5-7 km thick. In the Cariboo Mountains of east-central British Columbia rocks in the basal few km of the WSG comprise basin floor strata of the Kaza Group consisting of areally extensive sheetlike sandstones intercalated with thick, equally areally extensive mudstone (thin-bedded turbidite) sheets. In more detail, sandstone units are made up of one or more sand-rich architectural elements, including terminal splays, distributary channels, avulsion splays, scours, scour fills and feeder channels. Recently it has been suggested that while the dimensions and size ratio of basin-floor architectural elements is independent of basin size or tectonic setting, the distribution and abundance of those elements varies in regards to 4th order changes in the “graded” or “out-of-grade” condition of the sedimentary system.

The WSG has been interpreted to be a prograding passive-margin depositional system with a profile length measured in 100’s of km from shelf edge to basin floor and where vertical stratal changes occur on the several 100 m to several km scale. Owing to its thickness (>3 km), but admittedly poor chronological control, basin floor deposits of the Kaza Group most probably encompassed a number of 4th (and 3rd) order sedimentary cycles and also some number of graded and out-of-grade stratal conditions. Intuitively these changes should be manifest in the basin floor sedimentary record and reflected in the abundance and stratal stacking of its composite architectural elements. To date not such intercalation has been observed in rocks of the Kaza Group. Instead basin floor strata show a consistent km-scale (vertical) change from almost exclusively terminal splays deposited in the medial part of the lobe system to terminal splays with common distributary channel deposits in the more proximal basin floor. One possible explanation for this observation is that large-scale passive-margin systems, like the WSG, are generally in grade, and only temporally made to be out of grade. Conversely, smaller basin-floor systems, for example in tectonically active or salt-withdrawl mini basins, may remain either graded, or more importantly out-of-grade throughout their entire sedimentary history.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013