Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Differentiating Deep-Marine Overbank from Crevasse Splay Deposits in Outcrop: An Example from the Windermere Supergroup, Castle Creek, British Columbia
(1) Earth Sciences, University of Ottawa, Ottawa, ON, Canada.
At Castle Creek study area, thick sand-rich deposits intercalated with fine, thin-bedded distal levee turbidites are interpreted to be deep-marine overbank and crevasse splays. Although assumed to be similar to their better-known continental counterparts, significant differences in formative processes gives rise to important differences in stratal composition.
Overbank splays occur as two kinds: isolated and multiple bed complexes. Isolated beds are single, thick-bedded (40-200 cm), Tbcd or Tbd turbidites. The planar laminated division (Tb) is anomalously thick and comprises ~75-90% of the bed. Beds are tabular with sharp, planar and non-erosive bases. Multiple bed complexes consist of amalgamated, normally graded, coarse-grained structureless sandstone units 2-4 m thick. Multiple bed complexes are laterally continuous over several hundreds of meters, however, individual beds are laterally discontinuous, extending less than 100 m. Both isolated and multiple bed complexes have low (<10%) detrital mud content. Isolated beds were deposited by anomalously thick flows that overtopped the channel and deposited under competence-driven conditions. Multiple bed complexes represent a partial breach in the levee through which multiple flows escaped the channel and deposited under capacity-driven conditions.
Crevasse splay deposits consist of coarse-grained, poorly-sorted, coarse-tail graded, structureless sandstones with a high (30-50%) mud matrix content. Units range from 2-8 m thick and beds range from 5-125 cm thick. Two end member kinds are recognized: tabular and amalgamated. In tabular units, sandstones commonly contain a small number of large, isolated, tabular mudstone clasts. Beds generally show negligible change in facies or thickness laterally. Amalgamated units contain mudclasts that tend to be smaller (< 50 cm) but are more abundant, locally forming breccia layers. Also, bed bases are typically deeply scoured and beds cannot be traced for more than 150 m laterally. Mud-matrix-rich structureless sandstones reflect rapid, capacity-driven deposition possibly downflow of break in slope that formed a plane-wall jet with (hydraulic) jump. Immediately downflow of the jump deep scouring of the bed and rapid deposition from the collapsing sediment cloud deposited the amalgamated facies. Beyond the jump, locally-generated turbulence was damped by high sediment concentration, seafloor scour became negligible, and matrix-rich, tabular strata were deposited.