Abstract: Sequence Stratigraphy and Hydrostratigraphy in a Cratonic Basin: Outcrop Models from the Triassic, Southern Germany

Thomas Aigner

The classical Germanic Triassic represents a natural laboratory to study the stratigraphic dynamics in a typical cratonic basin. This paper has two goals: (1) to highlight the insights (and problems) gained by reviewing the Germanic Triassic in terms of sequence stratigraphy, and (2) to translate the observed patterns of basin fill into fluid-flow units relevant for modeling reservoirs and aquifers (hydrostratigraphy).

Sequence Stratigraphy

The sequence stratigraphic methodology can be applied to cratonic basin fills as well, although it has been developed in passive continental margins. The German Triassic represents a second-order transgressive/regressive cycle in which the continental red beds of the Buntsandstein pass gradually upward into the Muschelkalk carbonates (and evaporites), and back into the terrestrial Keuper. This second-order cycle is built by at least 13 third-order depositional sequences. Many bounding surfaces, such as sequence boundaries and (maximum) flooding surfaces, represent widely used marker beds, long used in classical lithostratigraphy.

A variety of parasequence types with distinct sedimentary and paleoecological changes, stacking patterns, and sequence anatomies is revealed by outcrop analysis. Cratonic basins show a number of differences to conventional sequence stratigraphic signatures, including the apparent scarcity of true lowstand deposits, the importance of flooding surfaces, the impact of subtle tectonics, and the influence of climatic changes on sequence development, especially in continental successions.

A regional coastal onlap chart can be constructed using a synthesis of outcrop and well-log data on stratal geometry, facies, cycle stacking patterns, and paleogeography. The observed cycles appear to correlate fairly well to those in other areas, but include a number of additional sequences not shown in the Haq et al. (1987) chart. Comparative analysis of regional onlap curves from different, globally spread Triassic basins, together with an improved biostratigraphy, will be necessary to relate the observed changes to eustatic vs. tectonic and climatic controls and to produce a refined eustatic chart. The German basin has potential to provide a calibration point for such an analysis.

Hydrostratigraphy

An additional major aim of this work is to assemble a set of outcrop analogs for both hydrocarbon reservoirs and for groundwater aquifers (environmental geology). The hierarchy of sedimentary "genetic units," as determined by sequence stratigraphic analysis, can be translated into "hydrostratigraphic units." The different scales of genetic sedimentary units determine different scales of heterogeneities for the hydraulics of fluid flow. Selected intervals were studied with a minipermeameter, and outcrop gamma-ray logging integrates surface with subsurface sections. In addition, georadar profiling helps delineate the three-dimensional facies architecture of fluvial sand bodies. The measurements at the outcrop analogs are aimed at constrain--the input parameters for reservoir and aquifer modeling.

AAPG Search and Discovery Article #90949©1995-1996 AAPG International Distinguished Lecturers