BORED AND ENCRUSTED CARBONATE COBBLES AND HARDGROUNDS AT THE BASE OF A REGRESSIVE SEQUENCE (LOWER SUNDANCE FORMATION, MIDDLE JURASSIC, EASTERN WYOMING)

The Stockade Beaver Shale Member (Callovian) of the lower Sundance Formation exposed near Hulett in eastern Wyoming contains numerous encrusted and bored carbonate hardgrounds and cobbles in its upper third. The enclosing shale is primarily light olive gray and very fine-grained, with a few thin rippled interbeds of calcareous siltstone and fine sandstone. These sediments were deposited in quiet offshore waters which experienced occasional storms washing in coarser sediments. Some of the calcareous silts were contemporaneously cemented on the sea floor, forming hardgrounds which were encrusted and bored by a variety of organisms. At the same time calcareous silts were piped down crustacean burrows and then cemented early as well. The cemented burrow fillings were exposed on the sea floor when their surrounding clay-rich matrix was winnowed away. The resulting cobbles were then encrusted by at least three bryozoan species, one serpulid species, and calcareous foraminiferans. The primary borings are Gastrochaenolites torpedo, which is produced by bivalves. The cobbles and hardgrounds commonly show only one or two generations of borings. The first set, usually concentrated on one side, is filled with cemented sediments and often truncated by abrasion. The second set of borings is usually on the opposite side of the cobbles, and they are generally well preserved and free of infilling sediment. The encrusters also show no more than two settling generations, and they tend to be at early stages of their development and distributed cryptically in the borings and on the sides of the cobbles. These cobbles apparently had a short residence on the sea floor, being overturned at most only once before final burial. They mark the transition between deeper low-energy conditions and the advance of shallower high-energy sands during a regional regression. The hard substrate fossil assemblage reveals that early cementation was a prominent factor in this transitional sedimentary environment, as was rapid sedimentation.