Early lithification of limestones in the Redwater Shale Member of the Sundance Formation (Jurassic) of southeastern Wyoming

Calcareous concretions, limestone cobbles, and limestone layers are present in the Redwater Shale Member (Jurassic: lower Oxfordian) of the Sundance Formation in southeastern Wyoming. Many of the typically ellipsoidal and discoidal concretions are scattered through the shale where they formed. Layers of limestone cobbles represent accumulations of reworked concretions, While exposed on the sea floor, they were penetrated by rock-boring pelecypods and encrusted by oysters and polychaetes. One of these cobble layers is represented throughout an area several tens of kilometers in diameter. It is consistently found at the boundary between the lower shale unit and the upper siltstone unit of the Redwater. Its cobbles represent several lithologic groups. This implies derivation from several different source beds, and the cobble layer probably represents a lag deposit on an erosion surface. The average size of the cobbles is about 12 cm x 8 cm x 4 cm. Other similar but less widespread layers are in the lower shale unit of the Redwater. Similar evidence for early lithification and intraformational episodes of erosion is provided by some limestone layers and large boulders of oolite at the base of the Redwater Shale Member at one locality, and bored layers of fossiliferous limestone near the top of the member at two other localities. These limestone layers are discontinuous and the surfaces of lateral truncation are bored by pelecypods, and in one case encrusted by oysters. The limestones became indurated and were freed from overlying sediments by wave erosion during the Jurassic. Wave action on the limestones produced irregular ledges and boulders. The exposed parts of the limestones were ideal substrates for boring pelecypods and oysters. Some large, flat boulders of fossiliferous limestone exhibit pelecypod borings on all surfaces, but are bored most extensively on the underside. This distribution of borings can be explained in two ways. Either the boulders were