Petrographic study of Ordovician and Pennsylvanian carbonate rocks of the Caballo Mountains, New Mexico, has shown that precise quantitative examination of limestones is useful in interpretating their conditions of deposition and diagenesis. Criteria are given for the recognition of pore-filling sparry calcite, which is characteristic of well sorted, cleanly washed limestones, and secondarily recrystallized calcite. Measurement of grain orientations in these limestones shows that elongate grains have distinct preferred orientations. This preferential orientation probably reflects the direction of the ancient tidal currents and may help to locate ancient shorelines, because the long axes of grains preferentially oriented by tidal currents are generally perpendicular to the nearest shoreline. The quantitative compositional study of a single Pennsylvanian limestone bed shows that samples of similar composition occur along trends parallel to the grain orientations of the same bed. The individual components of this bed also show a grouping parallel to the gross composition and the clastic grain orientation. These parallel trends were probably caused by the water movements of tidal currents. The presence of green algae, the sorting and rounding of clastic fragments, and the generally distinct linear fabrics indicate that these limestones were deposited in agitated, warm water, generally less than 10 meters deep, an environment quite similar to that found on the Bahama Banks today

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