Petrography and Diagenesis of a Half-Billion-Year-Old Cratonic Sandstone (Hickory), Llano Region, Texas

We studied the Hickory Sandstone Member of the Riley Formation, Central Texas, to assess the diagenetic history of a cratonic sandstone, which, in spite of its great age, remains porous and permeable where it has not been buried more than 1 km. Owing to its stratigraphic position between Precambrian basement and overlying, low-permeability carbonate rocks, the Hickory was a potential pathway for fluids expelled from the Ouachita orogen. Although sandstones contain 20 authigenic phases, they show no evidence of extensive hot-fluid metasomatism typical of some other basal Cambrian cratonic sandstones. Iron oxide and calcite are the only volumetrically important authigenic phases, although authigenic quartz is abundant in local silcretes and in deformation bands. Quartz cement fills most pores in sandstones buried deeper than 3 km. The Hickory Sandstone, as much as 150 m thick, comprises chiefly marine sandstone with minor mud rock, conglomerate, and limestone, plus an ironstone unit 25 m thick. Sandstone is subarkose, arkose, and diagenetic quartzarenite, in which most or all feldspar was lost diagenetically. Although sediment was derived ultimately from the Precambrian basement, some was reworked from fluvial deposits and eolian dunes. Syndepositional authigenic phases are cellophane as cement, grain coats, and nodules; glauconite; aragonite; and either goethite or berthierine, the latter of which apparently quickly altered to goethite. Inferred berthierine ooids and berthierine-replaced carbonate shells probably formed a few centimeters below the sediment-water interface in anoxic muds but were episodically exhumed and oxidized to goethite before ultimate burial. Authigenic phases introduced during the initial burial phase, which lasted until the end of Early Ordovician time, are K-feldspar, siderite?, calcite, chlorite, and possibly some quartz. During deepest burial, possibly during the Middle Pennsylvanian, quartz cement and replacement dolomite were introduced. Dolomite was partly replaced by ferroan dolomite, whereas early calcite was replaced by ankerite. Hydrocarbons or H2S generated at this time invaded parts of the Hickory and reduced ferric iron to yield pyrite and simultaneously bleached beds by removing iron-oxide grain coats from sandstones and mud rocks. Uplift and erosion from the Permian through the Jurassic brought the Hickory close to the surface in places until it was reburied by Cretaceous rocks. Influx of meteoric water permitted the precipitation of a late-stage, spotty calcite cement, chiefly in the form of pea-sized concretions. Meteoric water again invaded the Hickory when the Llano region was uplifted farther in the Neogene and became responsible for the formation of local ferricretes, silcretes, precipitation of minor kaolinite and opal, and the dissolution of about 5 percent feldspar and an indeterminate amount of calcite.In terms of mass balance, (1) calcium and some bicarbonate for calcite cement were probably derived from carbonate components in the upper Hickory and overlying carbonates; (2) iron oxide, partly syndepositional, was produced partly by oxidation of biotite and amphibole; (3) during diagenesis, silica for normal quartz cement was generated by intergranular pressure dissolution of detrital quartz; (4) silica for silcrete was derived from the weathering of Precambrian and Paleozoic rocks in the Llano region and was precipitated by meteoric water; and (5) most of the aluminum released from the late-stage dissolution of detrital feldspar left the system. In shallow-buried Hickory sandstones, the combination of primary and secondary pores yields excellent aquifer porosities and permeabilities, but healed fractures and deformation bands produce permeability compartments.