--> Kinematics of Past Tectonic Forces Inferred from Differential Sedimentation

AAPG Eastern Section Meeting

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Kinematics of Past Tectonic Forces Inferred from Differential Sedimentation

Abstract

By analyzing changes in thickness of stratigraphic intervals across major fault systems, syndepositional fault movement during the respective depositional periods can be inferred. Major thickness differentials across faults are interpreted as evidence of tectonic-related syndepositional fault movement. Additional sedimentation occurs within the increased accommodation space created by vertical displacements along faults. These areas of increased thickness are greatest along normal faults that are perpendicular to the extension direction, and decrease to zero along faults that are parallel to the extension direction. Regional extension directions and relative magnitudes through time can be interpreted by analyzing differential sedimentation patterns across populations of normal faults with varying strike directions.

Detailed subsurface mapping of six stratigraphic units was used to produce a map-view summary of interpreted fault movements from the Early-Middle Cambrian (Reelfoot Arkose) to the Early Mississippian (New Albany Shale) in the Rough Creek Graben and Northern Reelfoot Rift in the southern Illinois Basin. Given the set of faults that were active during the deposition of the Early-Middle Cambrian Reelfoot Arkose, the interpreted extension (σ3) direction is northwest-southeast. During this time, the Laurentian tectonic plate was separating from the Amazonian plate and the active spreading center that ultimately produced the Iapetus Ocean was shifting from the Blue Ridge rift to the Ouachita rift. The northwest-southeast directed strain vector interpreted from syndepositional fault movement corresponds well with this tectonic environment.

Extension continues throughout the deposition of the Middle-Late Cambrian Eau Claire Formation however, the vector of extension appears to rotate to a north-northeast to south-southeast direction by the end of that period. This extension direction appears to then diminish to zero during the deposition of the Late Cambrian–Early Ordovician Knox Group. Relatively minor fault offsets occurred throughout the Middle Ordovician–Early Devonian Period. During the Late Devonian, an intriguing pattern of fault offsets occurred on the northeastern and southern edges of the Rough Creek Graben. This deposition is contemporaneous with the Acadian Orogeny to the east, and is interpreted as a far-field tectonic effect of the west–southwest-directed tectonism.