ONSHORE TO OFFSHORE MODERN MARINE COMMUNITIES, AND CRETACEOUS PALEOCOMMUNITIES FROM THE WESTERN INTERIOR BASIN: ANALYSIS OF THE DIFFERENCES

Modern communities show an onshore to offshore decrease in genera, but only a slight decrease in species diversity. This is because genera that can live under dimly lighted to dark conditions make up for the decrease in diversity; deep water can be expected to have more species of smaller size per genus. Shallow water is the reverse, fewer species per genus but more and larger genera. Examples are drawn from the Upper Cretaceous Cyclothems within the Western Interior Basin (WIB). Maximum flooding was at the Cenomanian-Turonian boundary, and the Turonian-Coniacian boundary. Onshore, both intervals start off with high diversity faunas, in many cases involving genera equivalent to the modern fauna. In oxygenated waters, both go through a siltstone to silty shale phase, decreasing in size. But it is here the similarity ceases, and oxygen restriction sets in pervasively on the transgression and highstand in the Cretaceous. This is brought about by: (1) widespread dysoxia in the ocean basins, so the WIB starts out with dysoxic to occasionally oxygenated waters; (2) five oceanic anoxic events (OAEs), each lasting 0.5 - 3 Ma; (3) a sill at the southern end of the WIB in the vicinity of north Texas, another trending NE-SW in southern Colorado-northern New Mexico, and still a third one in northern Colorado. These sills resulted in isolation of large portions of the foreland basin, and restriction of open circulation to these parts of the seaway. This hindered extensive populations based on planktotrophic larvae. But these dysoxic zones were commonly populated by Inoceramidae, Ostreidae, and Lucinidae, which apparently had larvae that were tolerant of oxygen-restricted conditions. Some of the Inoceramidae grew to enormous sizes (3-4m), leading to the conclusion they had photosymbionts or chemosymbionts in oxygen-depleted waters. This was especially true for the giants, reaching 0.5 to over 3m in diameter.