SHOALING CYCLES IN THE CAMBRO-ORDOVICIAN GALLITAN FORMATION: NORTHERN WYOMING AND CENTRAL MONTANA

Two types of meter-scale cycles are recorded in the Gallatin Formation from northern Wyoming and central Montana. These are developed in mixed carbonate and shale in shallow shelf to shoreline deposits. Grainstone-rich shoaling cycles are abundant throughout the formation. These cycles consist of the following succession: (1) dark shale with starved grainstone ripples, (2) upward-thickening very thin to thin grainstone beds and shale, (3) flat-pebble conglomerate, and (4) thrombolite mounds. Sharp transitions from flat pebble and/or thrombolites into shale with minor grainstone represent marine flooding surfaces. The bases of flat-pebble beds are sharp and represent regressive shoreline incision and reworking. Thrombolites developed at the tops of these cycles appear in cases to be transitional through a thin interval of mixed flat-pebble with thrombolitic matrix that represents microbial colonization of a flat-pebble open-framework. Grainstone-poor cycles begin with dark shale with abundant small carbonate nodules. In some cases the shale contains thin grainstone beds. This grades into shale with very few, but slightly larger nodules. The cycles are capped by beds of reworked carbonate nodules. If these are shoaling cycles, then the nodule-rich shale would record rapid deepening, alluviation, and low mud accumulation rates. Upward decrease in the amount of nodules would reflect increased mud deposition that outstripped concretionary precipitation of carbonate. The presence of grainstone in some cycles in the lower shale and not in upper shale is problematic however. If these are deepening cycles, then the nodule conglomerate beds would be either transgressive lags or regressive beach deposits. Intraclastic beds from grainstone-rich cycles record reworking of thinly interbedded grainstone and shale, whereas conglomerate beds from grainstone-poor cycles reflect winnowing of nodules from nodule-rich mud.