Abstract

The last and most extensive Jurassic marine transgression in North America began in the Callovian, and reached its maximum limits during the Oxfordian. This eustatic event was concomitant with a period of relatively high rates of plate movements. The sea spread southward across the North American craton from the Arctic to a Jurassic paleolatitude of approximately 30° N. To the west, the Pacific plate was being subducted beneath the North American plate as early phases of the Sierra Nevada batholithic intrusions and related volcanics were emplaced, forming a magmatic arc complex. During Callovian time, arid conditions prevailed in the Western Interior sea with most siliciclastics entering the sea from the east and south. The western margin of the sea was characterized by carbonate deposition in a rapidly subsiding trough. By Oxfordian time, marine deposition was taking place in a series of arc-trench related basins in the western cordillera, extending from present-day Alaska to Southern California, and in a broad epeiric sea that covered much of the Western Interior as far south as present-day New Mexico. At this time the Gulf of Mexico basin to the southeast was being exposed to oceanic circulatory currents for the first time from the incipient Atlantic Ocean and possibly from the Pacific Ocean across present day Mexico. Faunal and lithostratigraphic data strongly indicate that the Western Interior sea and Gulf of Mexico were not connected, even during the maximum sea level stand. Limited marine exchanges probably existed between the interior sea and the arc-trench related basins to the west, however.

Analyses of the litho- and biofacies of the Western Interior sea that existed during Callovian, Oxfordian and Kimmeridgian times, show that complex distributions of paleoenvironments existed contemporaneously. Red beds, carbonates and evaporites characterize much of Callovian deposits. However, marine and shoreline siliciclastic sands and muds were deposited along the eastern margin of the sea in the present day Black Hills area. A major shift in depositional style in the Western Interior began during the Oxfordian as tectonic activities to the west produced siliciclastic source areas. Fluvial systems delivered siliciclastics to the sea from the west and southwest, and periodically, from local sources such as the Belt Islands of present-day central Montana. Cratonic siliciclastic sources may have continued supplying nearshore settings along the eastern margin of the epeiric sea as they shifted eastward during the Oxfordian. The western and central portions of the sea were dominated by marine sand and mud deposition. Storm-generated currents formed and maintained sandy bars and muddy swales within these shallow portions of the sea. Deeper portions of the sea and shallow areas remote from siliciclastic sources were dominated by siliciclastic mud and argillaceous carbonate deposition. However, aridity in the southernmost portion of the sea allowed for the persistance of red bed, carbonate, evaporite, and eolianite deposition from Callovian through Oxfordian times. Continued uplift of western siliciclastic sources, related to magmatic arc growth and associated tectonic activities, along with relatively low rates of subsidence, and possibly by lowering eustatic sea level, allowed nonmarine environments to prograde eastward from the edge of the sea beginning in Late Oxfordian time. During the Kimmeridgian, and continuing into the Early Cretaceous, non-marine deposition along with many areas of non-deposition and erosion characterized the Western Interior.