Rapidly changing styles of subsidence revealed by high-resolution mudstone allostratigraphy: Coniacian of Sweetgrass Arch area, southern Alberta and northern Montana
Marine mudstone of the Coniacian Muskiki Member was deposited by storms on a shallow-marine ramp across southern Alberta and northern Montana. The rocks range from the C. crassus crassus Zone of the upper lower Coniacian to the S. depressus Zone of the upper Coniacian; however, upper Coniacian strata are only a few metres thick at Kevin Montana and appear to be thin to absent beneath parts of the adjacent Alberta Plains, probably because of minimal contemporaneous subsidence. The Muskiki Member is partitioned into three packages by subtle bevelling unconformities, cut during regional tectonic tilting. The lower package indicates flexural subsidence in the NW, whereas the middle package indicates subsidence in the SW. The rotation of the axis of subsidence may indicate an along-strike shift of the region of active thickening in the orogenic wedge. The upper package is dominated by a NW-trending region of stratal thickening, forming a trough-shaped rock body 100 km wide, >250 km long, and up to 50 m thick, ...