QUASI-PLANAR-LAMINATED SANDSTONE BEDS OF THE LOWER CRETACEOUS BOOTLEGGER MEMBER, NORTH-CENTRAL MONTANA - EVIDENCE OF COMBINED-FLOW SEDIMENTATION

Lower-shoreface to shallow-shelf strata of the Bootlegger Member of the Lower Cretaceous Blackleaf Formation are characterized by interbedded sandstone and mudstone. Sandstone beds are characterized by a peculiar planar lamination showing a subtle although perceptible undulation; spacing-to-height ratios of the undulation are generally 100 or more. Typically the undulation shows no evidence of lateral accretion but only vertical aggradation, and as a result most beds consist of a single laminaset. Aspects of quasi-planar-laminated beds indicate single-event storm sedimentation, and paleocurrent data indicate offshore sediment transport. By its sedimentary characteristics and its similarity with a bed configuration generated in an experimental wave duct, quasi-planar lamination is produced by high-energy combined flows. This style of stratification should be common in the shallow-marine stratigraphic record. and its recognition should aid in interpreting high-energy, combined-flow depositional events.