ABDEL-WAHAB, ANTAR A.¹, EARLE F. McBRIDE², and KITTY L. MILLIKEN^2
1Department of Geology, Faculty of Education, Tanta University, Kafr EI-Sheikh, Egypt
²Department of Geological Sciences, University of Texas at Austin, Austin, TX 78731

Abstract: Shallow Diagenesis of the Cratonic Hickory Sandstone (Late Cambrian), Llano Region

The Hickory Sandstone is a porous marine sandstone situated between low permeability Precambrian basement rocks and Late Cambrian platform carbonates and was invaded by brines expelled from the Ouachita orogen during the Late Paleozoic. However, nearly all of the 20 authigenic phases in the Hickory seem to be the product of normal shallow burial diagenesis aided by multiple episodes of invasion by meteoric water.

In terms of mass balance: calcium and some bicarbonate for calcite cement were probably derived from carbonate components in the upper Hickory and overlying carbonates; iron oxide was partly syndepositional and partly produced by oxidation of biotite and amphiboles; silica for normal quartz cement possibly was generated by intergranular pressure dissolution of detrital quartz; silica for silcrete was derived from the weathering of Precambrian and Paleozoic rocks in the Llano region and was precipitated by meteoric water; most of the aluminum released from the latestage dissolution of detrital feldspar left the system. Wide disparity of IGVs in Hickory sandstones and the presence of exploded micas suggests there was local removal of calcite cement, but in amounts that cannot be quantified. The intensity of sutured grains shown by some sandstones is anomalous considering the modest burial depth of these samples. The large meteoric fluid flux the sands have undergone may have promoted dissolution of quartz at grain contacts.

The only minerals in the Hickory that suggest invasion by hot fluids from the Ouachita orogen are small amounts of dolomite and ferroan dolomite throughout the member and, at the contact with the basement, possibly feldspar replaced by K-feldspar in fractures in granite and in paleosols. Ouachita-age tectonics developed fault gouge, deformation bands, and fractured quartz and feldspar grains. Many fractured quartz grains were healed by authigenic quartz.

Hydrocarbons and H₂S invaded parts of the Hickory and reduced ferric iron to yield pyrite and simultaneously remove iron oxide grain coats from sandstones and mudrocks and bleached them.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas