Mechanisms for High-Frequency Cyclicity in the Upper Jurassic Limestone of Northeastern Mexico

JOHNSON, C. R., and W. C. WARD, University of New Orleans, New Orleans, LA, and R. K. GOLDHAMMER, Exxon Production Research, Houston, TX

The 520 m of Upper Jurassic Zuloaga Limestone exposed in the Sierra de Bunuelos in southern Coahuila comprise 118 cycles of peritidal carbonate rock deposited on a gently dipping ramp. Field studies with Fischer plots and time-series analysis suggest that a Milankovitchian glacioeustasy mechanism is inadequate to describe the Zuloaga cycles. Autocyclic progradation may have been the major influence on depositional cyclicity.

Depositional cycles in the Zuloaga Formation typically are a few meters thick and asymmetric with subtidal wackestone and packstone grading upward into subtidal grainstone or into intertidal stromatolites. Width of the carbonate ramp is estimated to have been about 150 km. Sedimentation rates for these peritidal carbonate environments apparently exceeded subsidence rates inasmuch as most of the carbonate platform remained near sea level during Zuloaga deposition. The area was tectonically quiescent during the late Jurassic.

Time-series analysis tuned to short-term precession cyclicity (18,600 years) shows the following major peaks: 465,000; 138,000; 104,000; and 81,000 years plus minor peaks at 57,000 and 51,000 years. These first three major peaks may suggest relationship to the superimposed 413,000-, 123,000-, and 95,000-year eccentricity cycles assumed for the Late Jurassic, but the match is not precise. Other peaks can not be explained by Milankovitch eustasy. Time-series analysis tuned to obliquity shows no relationship to Milankovitch cyclicity.

Autocyclic shoreline progradation is a feasible mechanism for producing the high-frequency cycles, as suggested by (1) poor correlation with predicted Milankovitch periodicity shown by time-series analysis, (2) little evidence of subaerial exposure, (3) development of complete peritidal cycles, (4) general progradational sequences within each third-order unit, and (5) absence of polar glaciation during Late Jurassic.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)