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Early Syndepositional Fault Control in a Deep-Water Channel-Overbank System: Implications for the Interpretation of Inverse Grading

Dykstra, Mason 1; Gamberi, Fabiano 2
1 Dept. of Geology and Geological Engineering, Colorado School of Mines, Golden, CO.
2 Istituto di Scienze Marine, Consiglio Nazionale delle Richerche, Bologna, Italy.

A deepwater channel system in the Sanguiniére sub-basin of the Alpine-related Eocene Annot Formation of Southeast France is under investigation to understand the role of syndepositional faulting on lateral changes in sandstone bed architecture and sedimentology. Detailed sedimentary logging, photomosaic interpretation, and laser-based correlation were used to develop an exceptionally high-resolution stratigraphic framework in the field area. Using this framework and the sedimentary logs, lateral changes in facies and sedimentary structures were examined to determine changes in turbidite architecture away from a channel axis into the proximal overbank. Syndepositional faults which exhibit incremental, small-scale movement (meter-scale) were a critical factor in the development of both the stratigraphic architecture, and also the detailed sedimentology of individual event-beds. Lateral changes in both facies architecture and grading within individual beds from axial zones dominated by normal-grading to marginal zones of inverse-to-normal grading were recorded. These lateral changes were controlled largely by small-scale accommodation along the syndepositional faults. Initially in a flow, lateral accommodation into these fault zones trapped the coarse fraction, while flow stripping allowed finer sediment to spill over further into the overbank. As the accommodation space in the first fault zone filled up (still by the same turbidity current), coarser and coarser sediment could begin to spill over the fault zone further into the overbank, resulting in an inverse-graded bed. Finally, when the accommodation space in the first fault zone was completely filled (still by the same turbidity current), all the flow could bypass further outboard of that fault zone, and grading would become normal again, resulting overall in an inverse-to-normally graded bed. Therefore, the same beds which are normally graded in the axis of the system become inverse to normally graded on the other side of each of the syndepositional faults. Because this architecture is recorded repeatedly in the vertical section, along a series of normal syndepositional faults outboard of the channel axis, we can confidently directly relate it to this process. These field data call into question the longstanding interpretation of inverse-to-normal grading as a hallmark of hyperpycnal flows.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009