Cores from the giant Whitney Canyon-Carter Creek gas field in western Wyoming show the nature of a siliciclastic filled karsted carbonate surface in an area with limited outcrop exposure. Regional outcrop studies by others document that a large carbonate platform was subaerially exposed during early Meramecian through Chesterian time across Wyoming, creating a karst surface on the Madison Limestone. From Chesterian to Morrowan time, a west-to-east marine transgression filled solution features with siliciclastic material of the Morgan (Amsden) Formation and equivalents. At Whitney Canyon-Carter Creek Field, the upper Mission Canyon Formation (Madison equivalent) contains this mixed carbonate and siliciclastic facies.
The uppermost 70 to 100 ft (21 to 30 m) of the Mission Canyon at Whitney Canyon-Carter Creek is a correlatable transition zone from preserved evaporite and dolostone beds beneath to the overlying siliciclastics of the Morgan Formation. Cores contain fractured and stylolitized carbonates that underwent solution collapse and brecciation due to dissolution and the subsequent filling of cracks and vugs by siliciclastic sands. Solution-collapse breccias in the cores have a sandstone matrix and dolostone clasts that are rounded, stained to a pinkish hue, locally capped by red clay drapes, and sometimes aligned as if deposited by a current. Breccias interpreted as tectonic in origin have only cement with no matrix and contain angular clasts with matching clast boundaries, i.e., fitted, indicating that fracturing occurred without any extensive dislocation of the clasts.
The mixing of carbonates and siliciclastics within brecciated intervals is significant because these observations suggest that the Mississippian karst plain and associated shoreline extended considerably further west than previously thought, especially when thrust belt structural positions are restored. Evidence presented here suggests that the maximum western shoreline position was at least 20 miles (32.5 km) west of the Utah-Wyoming border.
Figures & Tables
Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.