ABSTRACT: Tectonic Accommodation, Cycle Symmetries and Stacking Patterns of Mixed Siliciclastic-Carbonate Shelf Sequences, Jurassic, Paris Basin

GUILLOCHEAU, FRANCOIS, Centre National de la Recherche Scientifique, ULP, Strasbourg, France

Jurassic strata of the Paris basin are divisible into sequences of three temporal and spatial scales. The two larger scale transgressive-regressive (T-R) cycles are 20 to 40 m.y. and 3 to 10 m.y. in duration. The former apparently are controlled by changes in subsidence rates related to major kinematic reorganizations of lithospheric plates. The latter, intermediate period T-R cycles reflect changes in subsidence rates caused by movement of reactivated basement blocks.

These two long period cycles controlled the geometry, degree of symmetry, and stacking patterns of the highest frequency genetic sequences, which have durations of tens of thousands of years and thicknesses of a few meters to a few tens of meters. Each intermediate scale T-R cycle is composed of 5 to 10 genetic sequences. These genetic sequences record complete cycles of base-level rise and fall, and are equivalent to progradational/aggradational units explained by Walther's law. The 3-D geometries of genetic sequences were established through well-log correlations using stacking pattern analysis, and depositional environments within them were inferred from well-log signatures calibrated to cores and outcrops.

The degree of symmetry of the two larger scale T-R cycles changes as a function of the mean subsidence rate. Increased subsidence rate accentuates the preservation of sediment deposited during the transgressive or base-level rise phase of the cycles. Large-scale T-R cycles are increasingly asymmetric (transgressive dominant, regressive subordinate) with increasing subsidence rates, and increasingly symmetric or asymmetric (regressive dominant) with decreasing subsidence rates. As individual T-R cycles are correlated across tectonic blocks of different subsidence rates (based upon thickness per unit time), the symmetries of the cycles change following the tendency described above. This sometimes causes a shift in the stratigraphic position of base-level fall unconformity and base-level rise condensed section surfaces (or their stratigraphic equivalents) within the larger scale T-R cycles.

Symmetries of the shortest period genetic sequences also vary as a function of their position within the larger scale T-R cycles (i.e., base-level rise or fall phases), and as a function of changes in tectonically induced accommodation space. For example, open marine facies tracts are symmetrical to slightly asymmetrical (transgressive dominant) during base-level rise or transgressive phases of longer period cycles, and strongly asymmetrical (regressive dominant) during base-level fall or regressive phases. As open marine facies tracts within genetic sequences are correlated across tectonic blocks with lower subsidence rates, they are symmetrical during regressive periods, and more asymmetrical (transgressive dominant) during transgressive periods.

Because structural blocks moved differentially, there are cases where a seaward-stepping or vertically stacked pattern of genetic sequences on a slowly subsiding basement block is transformed into a vertically stacked or landward-stepping pattern on a neighboring, rapidly subsiding block. As a result, a condensed section (or base-level fall unconformity) on one block may not correlate exactly with a condensed section (or base-level fall unconformity) on an adjacent block.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)