Identification and micro‐stratigraphy of hyperpycnites and turbidites in Cretaceous Lewis Shale,Wyoming

Sandy hyperpycnal flows and their deposits, hyperpycnites, have been documented in modern environments and, more recently, in Cretaceous and Tertiary strata; they may be more common in the rock record, and within petroleum reservoirs, than has been previously thought. Muddy hyperpycnites also occur within the rock record, but these are more difficult to document because of their finer‐grained nature and lack of common sedimentary structures. This paper documents the presence of submarine slope mudstone and siltstone hyperpycnites (and muddy turbidites) in the delta‐fed, Upper Cretaceous Lewis Shale of Wyoming; based on field measurements, analyses of rock slabs and thin sections, and laser grain‐size distributions. Four lithofacies comprise laminated and thin‐bedded mudstones that are associated with levéed channel sandstones: (L1) grey, laminated, graded mudstone with thin siltstone and sandstone interbeds; (L2) dark grey to tan, laminated mudstone with very thin siltstone and sandstone stringers; (L3) light grey, laminated siltstones; and (L4) laminated mudstones and siltstones with thin sandstone interbeds. Two styles of mudstone grain‐size grading have been documented. The first type is an upward‐fining interval that typically ranges in thickness from 2·5 to 5 cm. The second type is a couplet of a lower, upward‐coarsening interval and an upper, upward‐fining interval (sometimes separated by a micro‐erosion surface) which, combined, are about 3·8 cm thick. Both individual laminae and groups of laminae spaced millimetres apart exhibit these two grain‐size trends. Although sedimentary structures indicative of traction‐plus‐fallout sedimentary processes associated with sandier hyperpycnites are generally absent in these muddy sediments, the size grading patterns are similar to those postulated in the literature for sandy hyperpycnites. Thus, the combined upward‐coarsening, then upward‐fining couplets are interpreted to be the result of a progressive increase in river discharge during waxing and peak flood stage (upward increase in grain‐size), followed by waning flow after the flood begins to abate (upward decrease in grain‐size). The micro‐erosion surface that sometimes divides the two parts of the size‐graded couplet resulted from waxing flows of sufficiently high velocity to erode the sediment previously deposited by the same flow. Individual laminae sets which only exhibit upward‐fining trends could be either the result of waning flow deposition from either dilute turbidity currents or from hyperpycnal flows. The occurrence of these sets with the size‐graded couplets suggests that they are associated with hyperpycnal processes.