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Combined Wave and Gravity-Driven Sediment Transport: Controls on Lateral Facies Heterogeneity on Subaqueous Muddy Shelf Clinothems


Production surveys demonstrate that significant petrophysical heterogeneity exists between mudstone intervals and even laterally within single beds. These findings are inconsistent with the concept of vertically accreting, laterally homogeneous basinal mudstones; suggesting that not all of the small scale geological variability that exists in the rock record can be explained by sediment composition or by burial-diagenetic history. The relationship between small-scale heterogeneity of mudstone fabric imparted by high-energy erosive events and biological reworking is one little considered, but potentially significant geological control on the petrophysical variability of mudstones, including pore aperture size distribution, mineral surface area and permeability. Detailed sedimentological and ichnological analyses were carried out on muddy shelf clinothems from the modern SW Louisiana (SWLA) Shelf and the Ordovician Power Steps Fm. (Bell Island Group, NL, Canada). On both clinothems grain-size trends, geometry of bounding/erosional contacts, and ichnological characteristics were recorded. Preliminary data reveal that in both examples topset architecture consists of cm-thick, normally-graded, laterally continuous, erosionally-based, cross-laminated silty to sandy beds; indicating offshore-directed sediment transport, sedimentary by-pass and little net deposition. In both cases clinothem progradation is interpreted to be facilitated by large scale slope failure and sediment gravity flows. Topset muds on the SWLA shelf exhibit a low-diversity ichnological assemblage consisting of mm-thick, vertical, unlined traces that cross-cut fewer than 2 to 3, cm-thick beds. In the Power Steps Fm. a low-diversity ichnological assemblage consisting of vertical to curved, dm-long traces cross cuts several dm-thick event beds. Fore- and bottom-sets in both localities exhibit dm-thick, laterally continuous beds with erosive, even bedding contacts. Modern muds on the SWLA shelf are moderately bioturbated and comprise cm-thick, vertical unlined traces that cross-cut multiple depositional layers. Fore- and bottom-sets in the Power Steps Fm. are unbioturbated. Ongoing research efforts will integrate the observed facies heterogeneity into a predictive, chronostratigraphic framework that will help to refine models of mudstone facies variability and improve our understanding of how physical and biological seabed processes control multi-scale mudstone heterogeneities.