SEDIMENTOLOGICAL AND STRATIGRAPHIC ANALYSIS OF CAMBRIAN AND ORDOVICIAN GALLITAN FORMATION, WYOMING AND MONTANA

Sedimentological, stratigraphic and conodont biostratigraphic data are presented for six sections across northern Wyoming and southern Montana. Biostratigraphic data indicate nearly complete coverage of the Cambrian–Ordovician boundary interval. Conodont zones ranging from Eocononodontus to Rossodus manitouensis are represented in these sections. Cryptic unconformities in some of these sections are represented by missing conodont zones/subzones. These unconformities are also detected in numerous carbon isotopic chemostratigraphic curves. Differences in lithofacies compared to equivalent age rocks in Colorado include higher percentages of shale and flat pebble conglomerate, indicating deeper inner detrital belt environments. Slump beds are commonly associated with hummocky cross-stratified grainstone facies. These cut across bedding and contain clasts that indicate derivation from the thinly laminated grainstone. Onlap of grainstone beds onto the slumps indicate that these were surface failures of partially lithified grainstone. The absence of shale and internal structures indicates deposition along a storm-dominated shoreface. Transitions were noted from buckled and slumped grainstone beds to flat pebble conglomerate beds. The flat pebble beds contain abundant matrix of coarse echinoderm-rich grainstone. The echinoderm debris was carried landward from deeper water and mixed with the flat pebbles during storm events. Slumped and buckled beds contain microdomains of vertically imbricated clasts, whereas the laterally associated flat pebble beds show dominantly sub-horiizontal imbrication. Failure of the shoreface likely took place as a result of cyclic wave loading associated with storms. This led to slumping, sediment mixing and eventual development of flat-pebble conglomerate. Storm-generated failures may be an important mechanism for generation of flat pebble conglomerate beds in the rock record.