Facies- and Position-Dependent Diagenesis in an Upper Cretaceous Deltaic Sand Body

Martin R. Schroeder, Thomas M. Lehman

Tributaries of Tornillo Creek in Brewster County, Texas, expose a cross section through an Upper Cretaceous deltaic sand body in the Aguja Formation. Along the axis of the sand body, distributary-channel and mouth-bar deposits are 15-20 m thick and are underlain by delta-front sandstones and prodelta shales. Traced laterally about 2 km, the sand body thins to less than 4 m of mouth-bar and subaqueous levee deposits interbedded with lignite. The sandstone cements are clay and iron oxide coatings on grains (Cl), quartz overgrowths (Q), kaolinite (K), calcite (C), and ferroan calcite (FC). Dissolution of plagioclase has created abundant secondary porosity. The interior of the sand body is cemented largely by Cl and Q precipitated early diagenetically. The mouth-bar and levee deposits are cemented by K, which plugged primary and secondary porosity remaining after Q. The lower and upper portion of mouth-bar and distributary sands and all delta-front splay sands are tightly cemented by C and FC. Minor differences in grain size and mineralogy between facies suggest that the distribution of cements was largely controlled by position within the sand body. Kaolinite may have precipitated preferentially in thin levee sands due to acid-leaching in proximity to lignites. Calcite may have precipitated by vertical invasion of fluids derived from shales that overlie and underlie the sandstone. Burial depth was only 1,300-1,900 m, but the elevated geothermal gradient in the Trans-Pecos region has yielded diagenetic features characteristic of more deeply buried sandstones f the Gulf Coast.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.