Cretaceous Marine Cycles of the Western Interior

The vertical sequences of lithologies and faunas in the center of the basin clearly depict all developmental stages of two major cycles of sedimentation, with lesser fluctuations producing major and minor alternation of sequential events (interbedding). Close repetition of lithologies and faunas, as defined by genera and morphotypes, from one-half cycle to the next, and between cycles, demonstrates environmental cyclicity in the basin during its-Cretaceous history. This suggests that the system may be modeled, utilizing all parameters, from the central basin area and that this model may be applied to the solution and prediction of paleoenvironments and sedimentary-faunal sequences in unstudied parts of the basin distant from the center, and in other similar basins. These initial interpretations are general and subject to scrutiny, and many aspects of the problem must be investigated further, but they do provide a base for further work in greater detail and in other areas. Four major marine and marginal marine basin environments can be defined: (A) Mixed coastal plain fluvial and estuarine, restricted bay, lagoon, and salt pond-marsh environments in well agitated to quiet waters, and depths less than 100 feet. (B) Beach-dune, marine bar, delta, and forebeach sand sheet environments in waters of normal salinity and depths usually less than 50 feet. (C) Mid-shelf clay belt consisting of dark muds of the central basin area represent depths of 100 to 200 feet for a certain lithology and 300 to 500 feet for other lithologies. (D) Central basin carbonate belt consisting of various carbonate lithologies were probably deposited within reach of active currents periodically and within the photic zone most of the time, and were deposited at depths of 100 to 200 feet. A generalized interpretation of the data suggests that the basin evolved in the following manner. Initial flooding and terminal regression produced a very shallow water suite of inner sublittoral and marginal marine clastic environments along the basin axis. Possibly with isolated basins periodically developing in an area not greater than 300 miles wide. Thick clastic sedimentary sequences were produced in a wedge just offshore from the strand producing mid-basin subsidence. Continued flooding caused migration of the strand, the clastic belt, and the thickest wedge of sedimentation east and west, and deepening of the central basin area to as much as 500 feet, with the basin about 700 miles wide, during middle transgression. The now deeper central basin area received a thinner sequence of sediments because it lay offshore from the main wedge of clastic sedimentation. At maximum transgression, the coarse clastic and clay belts continued their eastward and westward migration as the basin widened to 900 miles. The center of the basin began to rise, producing a 100 to 200 foot deep bank dominated by carbonate sedimentation, and separated from the equally shallow outer edge of the nearshore clastic belt by a deeper trough of clay sedimentation to the west and partially to the east. Regression of the seaway, which produced a reverse sequence of events in the central basin, was marked by slight mid-basin subsidence as the principal centers of clastic sedimentation moved toward this area, and ultimately by filling with clastics and shoaling as the basin shrank in size and the sea withdrew.