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McBRIDE, E. F., University of Texas, Austin, TX, M. DANE PICARD,* University of Utah, Salt Lake City, UT, and R. L. FOLK, University of Texas, Austin, TX

ABSTRACT: Elongate Concretions in Sandstone: Evidence of Ancient Flow Directions in Groundwater

Highly elongate, calcite-cemented concretions that show remarkably uniform orientations occur in friable sand of the fluvial Zia Formation (Miocene) in New Mexico, southwestern United States, and in an unnamed shallow-marine formation (Pleistocene) in Basilicata, southern Italy.

The Zia concretions are Previous HittaperedTop cylinders from 30 to 10 m long with aspect ratios of 10:1 to 20:1. They show no selectivity to stratification types and have no pattern of distribution in outcrops. The standard deviations of long axes of concretions for two localities with 28 and 32 concretions, respectively, are less than 12 degrees . (delta)13C and (delta)18O values of calcite cement range from -7 to -4 o/oo (PDB) and from -13 to -11 o/oo (PDB), respectively. The concretions are composed of aggregates of spherical microconcretions the size of peas and grapes that internally have poikilotopic fabric.

Basilicata concretions resemble pencils and cigars and reach 50 cm in length. They occur singly and in laterally-linked bodies. Most are in horizontally laminated beds; a few are in cross-stratified beds. Concretions occur in planes that were horizontal or nearly so and in places cut across bedding. The standard deviations of long axes of each of six localities with more than eight measured concretions are less than 7 degrees ; the standard deviation of the average orientation of concretions from 30 localities over a 200-sq km area is less than 17 degrees . (delta)13C and (delta)18O values of calcite in the sands range from -10 to -1 o/oo (PDB) and from -5 to -3 o/oo (PDB), respectively. The isotopic values reflect a mixture of calcite cement and approximately 25% detrital marine skel tal grains. Calcite cement formed chiefly as overgrowths on skeletal grains, and lacks the microconcretionary form typical of the Zia concretions.

The isotopic values, orientations, and textures of the concretions in both formations indicate that they were formed by calcite precipitated from meteoric groundwater. The long axes of concretions depict paleoflow paths of the groundwater-toward the coast for the Basilicata sand and toward the fluvial drainage for the Zia Formation. The planes of concretions in Basilicata suggest the position of ancient water tables or interfaces of different water masses. Calcite cement was derived internally from skeletal grains. In the Zia Formation, the sands were cemented by carbonate derived from an unknown source outside the formation. The microconcretions are examples of the shelf-organizational behavior locally shown by calcite cement.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.