precision stratigraphy associates


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Albian rudist biostratigraphy (Bivalvia), Comanche shelf to shelf margin, Texas

SCOTT R.W., CAMPBELL W., HOJNACKI R., WANG Y. & LAI X. (2016).- Albian rudist biostratigraphy (Bivalvia), Comanche shelf to shelf margin, Texas.- Carnets Geol., Madrid, vol. 16, no. 21, p.513-541.

Rudists were widespread and locally abundant carbonate producers on the Early Cretaceous Comanche Shelf from Florida to Texas, and on Mexican atolls. As members of the Caribbean Biogeographic Province, their early ancestors emigrated from the Mediterranean Province and subsequently evolved independently. Comanchean rudists formed biostromes and bioherms on the shelf interior and at the shelf margin. Carbonate stratigraphic units of the Comanche Shelf record rudist evolution during the Barremian through the Albian ages and an established zonal scheme is expanded. This study documents new Albian rudist occurrences from the Middle-Upper Albian Fredericksburg and Washita groups in Central and West Texas. Rudists in cores at and directly behind the shelf margin southeast of Austin and San Antonio, Texas, complement the rudist zonation that is integrated with ammonites and foraminifers. These new rudist data test long-held correlations of the Edwards Group with both the Fredericksburg and Washita groups based solely on lithologies. Rudist and foraminifer biostratigraphy indicate that the Edwards Group is coeval with the Fredericksburg not the Washita Group. (read more)

Barremian-Aptian-Albian carbon isotope segments as chronostratigraphic signals: Numerical age calibration and durations


Chemostratigraphic signals are becoming common tools in chronostratigraphic correlations. Specifically total organic and carbonate carbon percentages (TOCorg/carb) and carbon isotope curves are diagnostic in Cretaceous Aptian-Albian-Cenomanian strata. Cretaceous carbon isotope signals define oceanic anoxic events (OAEs) and have been subdivided into successive segments. These segments are used as correlation markers because they represent oceanographic and depositional conditions, and hence, incur time significance. Although the segments have been integrated with biovents in key sections, numerical ages of the segments have yet to be calibrated. Because direct correlation of these segments of carbon isotope curves with interbedded radiometrically dated beds is not possible currently, other methods must be applied to interpolate numerical ages. (read more)

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