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Abstract: Fluvial Architecture, Cycles, and High-Resolution Stratigraphy in Apparently Homogeneous Braidplain Deposits

ANDERSON, DONNA S., and TIMOTHY A. CROSS, Genetic Stratigraphy Research Program, Department of Geology and Geological Engineering, Colorado School of Mines, Golden, Colorado.

Thick successions (each up to 90 m thick) of dominantly trough cross-stratified, fine to medium sandstone crop out along the northern margin of the Hornelen Basin (Devonian) of western Norway. Analysis of facies relationships within these and interfingering alluvial-fan and lacustrine strata suggest deposition on a braidplain. Relatively constant grain size, notably sparse pebbles, and lack of mudstone create indistinct lithologic contrast throughout these successions. Therefore, analysis of fluvial architecture and stratigraphic cycles relies primarily on measuring attributes of a low diversity of sedimentary structures and of channelforms bounded by low-relief scour surfaces. Important attributes consist of trough-set thickness, degree of soft-sediment deformation, and channelform architecture and thickness. These types of measurements are applicable to analysis of whole core and borehole imaging data.

At the largest and smallest scales of stratigraphic observation, fluvial sandstone successions display high degrees of organization but disparate degrees of correlatability. At the smallest scale, they are organized into broad, sheetlike channelforms ranging from 1 to 8 m thick and >200 m wide with poor lateral correlatability. At the largest scale, easily correlatable 30 to 90 m thick successions extend >10 km from the basin margin to center and are interbedded with lacustrine and alluvial-fan strata as stratigraphic “megacycles.” An intermediate scale of organization is only apparent by quantifying preservation tendencies and proportionally changing facies successions within and among channelforms. Intermediate-scale cycles are correlatable at a scale useful for analyzing reservoir compartments within homogeneous-appearing sandstone successions.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah