Foraminiferal biostratigraphy and petrography of the Lower Pen Formation (Upper Cretaceous), Big Bend Region, Trans-Pecos Texas

In the Big Bend region of Trans-Pecos Texas, Upper Cretaceous pelagic marine limestone of the Boquillas Formation is overlain by marine shale of the Pen Formation. This transition from predominately carbonate sedimentation to clastic sedimentation occurred diachronously across the interior of North America where it is recorded, for example, at the contact between the Niobrara Chalk and Pierre Shale and between the Austin Chalk and Taylor Marl. In the Big Bend region, this transition interval is sharply defined to the east but gradational to the west and consists of an interbedded series of thin pelagic limestone alternating with marl beds. The limestone beds are progressively thinner and more discontinuous upward within the sequence, and gradually pinch out to the west. Typically limestone beds vary from well-laminated foraminiferal wackestones to crudely laminated foraminiferal-ostreid-inoceramid packstones. Marl beds are texturally similar to the well-laminated foraminiferal wackestones, but contain substantial terrigenous clay in addition to microcrystalline calcite. Lamination within the limestone and marl beds is defined by foram-rich and foram-poor layers. The entire stratigraphic sequence is characterized by large shells and shell fragments of the "giant" inoceramid Cordiceramus sp., many of which are preserved as entire specimens in living position. The foraminiferal fauna is diverse, and consists primarily oi Gaudtyina {Siphogaudiyina) austiniana, Globigerinella aissana, Gyroidina depressa, G. globosa, Heterohelix reussi, Lenticulina rotulata, Marginotruncana coronata, and Rositafornicata. These taxa indicate that the Boquillas/Pen contact interval spans the Globotnmcana bulloides Assemblage Zone, composed of the Marginotruncana concavata Subzone and Globotnmcana fornicata Subzone, of Pessagno (1967), and is Santonian in age. The rhythmic alternation of limestone and marl possibly recorded wet-dry climate cycles driven by Milanko\ itch orbital variations that resulted in variable clastic sediment dilution and carbonate productivity, Inoceramid-produced biological effects surrounding inoceramid bioherms and/or irregularities in sea floor topography probably led to increased discontinuity of limestone relative to marl beds.