Sea Level Changes Reflected in the Carbon Isotopic Composition of Sandstone Cements Frio Formation, Texas

Nicholas B. Harris

The Frio Formation is an Oligocene clastic sequence in the western Gulf of Mexico. At St. Charles Ranch field, Texas the Frio is buried from 2300 to 3300 meters depth; it consists of interbedded shelf sandstones and shales. Upper and Lower Frio sandstones were deposited in relatively shallow water, indicated by hummocky cross stratification and ripple lamination. Middle Frio sandstones display bioturbation and lack physical stratification, indicating deeper water deposition.

Calcite is the major cement in the sandstones. Calcite is also present in shales as micritic grains and microfossils. Two limestone beds, with ^dgr¹³C of -20^pmil_(PDB), probably originated as methane seeps on the seafloor. The ^dgr¹³C of calcite cements in sandstones and detrital calcite in shales parallels the sea level cycle indicated by the depositional character of the sandstones. Sandstone cements and shale calcite in the shallower water Lower Frio have ^dgr¹³C of approximately 1 to -5.5^pmil, near normal seawater. The ^dgr¹³C composition of calcite decreases upward as depositional water depth in the Middle Frio increases, reaching a minimum of -4 to -7.5^pmil. As depositional water depths decrease in the Upper Frio, ^dgr ¹³C compositions return to their near-seawater value.

The systematic change in ^dgr¹³C is best explained by cold, deep, ¹³C-depleted water impinging on the continental shelf during high sealevel. Calcite compositions were further modified by incorporation of light carbon from methane seeps. Alternatively, the contribution of seep-derived carbonate was systematically affected by sealevel, possibly due to the loading and unloading of the continental shelf.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995