--> Application of Chemostratigraphy to Differentiating Bounding Stratigraphic Surfaces, by R. A. Eisenberg and P. M. Harris; #90986 (1994).

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Abstract: Application of Chemostratigraphy to Differentiating Bounding Stratigraphic Surfaces

R. A. Eisenberg, P. M. Harris

Outcrops of the Permian San Andres Formation along the western edge of the Guadalupe Mountains of southeastern New Mexico have been thoroughly studied from a stratigraphic perspective. In spite of these intensive studies, numerous questions remain regarding the correlation and hierarchy of important regional bounding stratigraphic surfaces (sequence boundaries, cycle boundaries, and flooding surfaces.) Utilizing multielement geochemistry, we have further examined five bounding surfaces within the middle and upper San Andres Formation.

Stratigraphic intervals were sampled from two cored wells. Major, minor, trace and REE were measured on 44 interval composites, broken out by lithology, using a combination of techniques including inductively coupled plasma and mass spectrometry. Significant variations of selected elements are observed when plotted against depth and lithology/texture across the five bounding surfaces. Principal components analysis, performed on centered log ratios for 25 chemical variables, yielded five principal components with eigenvalues greater than one that explain 75% of the data variance. Clustering of variables was investigated through varimax rotation of the principal components (factor analysis).

Three factors related to depositional/diagenetic processes were interpreted and used to evaluate the bounding surfaces:

(1) A "detrital clay" factor that loads on Si, Al, Fe, Ni, As, and Ti, which we relate to regional subaerial exposure with weathering and transportation of fine-grained clastics across the carbonate shelf; (2) An "organo-metallic" factor that loads on organic carbon and base metals, that we relate to flooding and reworking of organics from the up-dip portions of the shelf (terrigenous carbon) and improved circulation in the basin (marine carbon); and (3) A "dolomitization" factor that loads on Ca and Mg and is related to various dolomite types and calcite cement content.

Relatively high concentrations of elements associated with the "detrital clay" factor occur within two cycle boundaries coincident with the transition from the middle to upper San Andres and may represent a large-scale sequence boundary. Concentration of organic carbon and associated base metals coincides with a flooding surface and occurs within the middle San Andres at the transition from outer ramp to cyclic ramp-crest deposits.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994