Abstract

Carbonate nodules which occur within the trough cross-stratified highly glauconitic Cambrian Lion Mountain Sandstone of central Texas are primarily an expression of depositional fabric. Trilobite and brachiopod skeletal material accumulated as "shell lag" deposits within troughs in front of migrating, subaqueous megaripples in a tidal inlet and form the framework of the nodules. High depositional porosity, due to the morphology of the skeletal material, was filled, at least partly, by passively precipitated acicular to bladed spar while the sediment was still at or near the sediment-water interface. Clastic constituents rest on the upper surfaces of large skeletal grains in geopetal fashion; consequently, the calcite cement is best developed as downward-elongated spar crystals on the undersides of the skeletal debris. Large areas within the nodules are now filled with radiaxial, fascicular-optic, and planar cross-twin lamellae calcite cement as a result of neomorphism of the original acicular spar. All three types of spar occur as length-fast and length-slow calcite.

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