STRATIGRAPHY AND SEDIMENTOLOGY OF THE CAMBRIAN AND ORDOVICIAN GALLITAN FORMATION, WYOMING AND MONTANA

The Upper Cambrian and Lower Ordovician Gallatin Formation of Wyoming and the Montana consists of mixed siliclastic and carbonate deposits the inner detrital belt of western Laurentia. Conodont biostratigraphy indicates that remarkably complete stratigraphic successions across the Cambrian–Ordovician boundary interval. The Gallatin Formation exhibits two facies associations: a grainstone-dominated association deposited above fair weather wave base and a shale-dominated association dominated below fair weather wave base. Hummocky cross-stratified grainstone in both facies associations indicates that deposition occurred on a storm-dominated setting. Both facies associations include a variety of slump and slide features including buckled grainstone beds. Transitions were noted from buckled beds to flat pebble conglomerate beds. Slumped and buckled beds contain microdomains of vertically imbricated clasts, whereas the laterally associated flat pebble beds show dominantly sub-horiizontal imbrication. The flat pebble beds contain abundant matrix of coarse echinoderm-rich grainstone. Sedimentological and stratigraphic relationships suggest that buckled beds formed during storm events due to shoreface slope failure of grainstone beds. Buckling and downslope movement of broken, but semi-lithified or consolidated grainstone beds resulted in chaotic to steeply inclined packing of clasts with relict bedding. Downslope acceleration continued until relict bedding completely disarticulated. . The echinoderm debris was carried landward from deeper water and mixed with the flat pebbles during the storm events. A resulting mass flow moved dominantly by laminar shear, mixing in bioclastic debris transported during the storm from deeper water. This resulted in the formation of flat-pebble conglomerate beds with sub-horizontal clast orientations.