Late Albian-Early Cenomanian Flooding History: Southern U.S. Western Interior

An integrated study of outcrop and well sections in central and southeastern Colorado, northeastern New Mexico, and northwestern Oklahoma, reveals three thin sequences comprising the early part of the Greenhorn third-order cycle (Late Albian-Early Cenomanian). Sedimentological, micropaleontological, palynological, and organic geochemical data demonstrate three flooding events from the south. Environmental details of the marine-terrestrial transitions during a continental flooding event show how they fit into the big picture of the first-order Zuni Sequence. Each of the three sequences (referred to as sequences 3.1, 3.2 and 4) records biofacies shifts of over 200 km within vertical sections of less than 20 m that mark ephemeral Tethyan flooding into southeastern Colorado. In each sequence, basal fluvial-paralic sandstone with non-marine fossil assemblages, isotopic and palynofacies signals passes vertically into a section of marine-influenced shale and sandstone. Marine fossils, mainly moderately diverse agglutinate foraminiferal biota, nearshore dinoflagellate cysts and acritarchs, and Skolithos ichnofauna, become progressively poorer up dip until only brackish-tolerant ichnofauna, few agglutinate foraminifers, and non-marine palynomorphs remain. Two of the three thin sequences record previously unrecognized transgressions that appear to have lasted no more than a million years each, resulting in ephemeral biotic connections across the shelf, and had high amount of fresh water input. Sequence 3.2 briefly connected with the Mowry Sea. Therefore, the database generated in the study area covers a considerable portion of the southern Western Interior Basin and provides a model of transgressions across low-relief continental epeiric seas.