Depositional Response to Structurally-Generated Topography in Deep Water Thrust Belts: Comparison of the Grès d'Annot Basin (Southeast France) with Modern Deepwater Fold and Thrust Belts Gillian M Apps and Frank J Peel, BHP Billiton Petroleum BHP Billiton Petroleum, 1360 Post Oak Blvd, Houston, TX 77056. Gillian.Apps@BHPBilliton.com Seismic examples The seismic example shown below is of a turbidite-dominated sequence deposited over an active thin-skinned thrust belt. The stratal patterns seen on this data exactly match the prediction derived from geometric forward-modeling. Predicted stratigraphic patterns over an active thrust The effect of an active syndepositional thrust on the geometry of the overlying sediments can be easily modeled. These simulations were created in Microsoft Excel using the application Franks_Fault. For this simple thrust geometry, the stratal response depends on whether the rate of sediment aggradation is greater than the rate of local structural uplift. In the left hand model, the uplift rate is relatively large, outpacing the rate of sediment aggradation. The predicted stratal response consists of progressive offlap on the back of the structure coeval with progressive onlap on to the front. The position of the onlap and offlap shift relative to the sediment sequence . In the right-hand model, sediment aggradation rate is greater than the rate of local uplift. The predicted response consists of a shifting zone where the sediment is relatively thin. In this setting, the intrabasinal high would probably have minor surface expression, but its effect on the sediment architecture is significant. Abstract Thin-skinned thrust ramp anticlines that grow during deposition in a deepwater setting should produce a predictable stratal response. An area of seafloor is uplifted over the footwall ramp, on to which the deepwater sediments thin or pinch out. As the thrust sheet moves forwards, the positions of sediment thinning or pinchout migrate backwards across the thrust sheet, producing a characteristic onlap/offlap pattern. Seismic data from ramp anticlines in the Niger Delta toethrust system show stratal patterns which closely match the geometries predicted by forward modelling. The Grès d'Annot Formation of SE France is a well-exposed turbidite system that was deposited in the Eocene-Oligocene Alpine foreland basin. The basin floor experienced active thin-skinned folding and thrusting during deposition of the turbidite sequence. Therefore the turbidite sequence should display some of the features predicted from structural modelling and observed in analog seismic data sets. This presentation will show some features that we believe are related to syndepositional thrusting. For example, the base and top of the turbidite sequence commonly consists of a distinct thin-bedded silty facies that probably represents onlap and offlap against a tilted substrate, which may in places be related in part to movement of the basin floor over deeper thrust ramps. Some major channel incisions may be a response to local uplift over thrust-related uplifts. Sediment supply variations, eustacy and other extrinsic factors also influence the stratal geometries within the basin, but the role of syndepositional thrusting within the basin may have been previously underestimated. Offset onlaps over a ramp anticline data courtesy of Mabon Inc., shown with thanks slow aggradation model rapid aggradation model initial geometry thinning-up offlapping interval non-deposition thickening-up onlapping interval forward model of structural and stratigraphic evolution normal thickness chronostratigraphy of the syntectonic sediments normal thickness normal thickness thickening-up onlapping interval thinning-up offlapping interval non-deposition relatively condensed section intrabasinal high; minor surface expression detail – backstepping offlaps and onlaps normal thickness thinning-up offlapping non-deposition normal thickness thickening-up onlapping normal thickness thrusting direction relatively minor perturbations of the thrust geometry produce even more complex stratal geometries, which might be mistaken for depositional features For example, these features developed above an active thrust set could be mistaken for sediment waves if the deeper structure were not recognized. enlargement of final geometry Gillian Apps and Frank Peel BHP Billiton, 1360 Post Oak Blvd, Houston, TX 77056

Depositional Response to Structurally-Generated Topography in Deep Water Thrust Belts: Comparison of the Grès d'Annot Basin (Southeast France) with Modern Deepwater Fold and Thrust Belts The setting of the Gres d’Annot turbidite system The Gres d’Annot turbidites were deposited in the SW Alpine foreland basin. The floor of the basin was affected by active thin-skinned thrusting throughout deposition of the Tertiary foreland basin fill. The nature of the basin and its sediment fill are well documented, as is the relationship of the basal turbidite onlap pattern to the underlying thrusts. However, the relationship of the stratal architecture of the internal parts of the Gres d’Annot to the thrusts has been the subject of previous study. gross stratigraphy of the SW Alpine foreland basin An example of one stage of sequential restoration is shown above. The thickness variations, stratal geometries and facies patterns within the Tertiary allow us to pull back the movement over a deep thrust ramp. The sediment record demonstrates that the deep thrust was active during the deposition of the older Tertiary sediments, and subsequent to the deposition of the Gres d’Annot turbidites. There is no reason to expect that thrusting conveniently ceased during deposition of the turbidites, so we would expect to see some record of it in the turbidite sediments. However, because the turbidites were deposited relatively rapidly, the geometric evidence of syndepositional thrusting should be subtle (consisting of minor thinning and gentle changes of internal dip). The orientation of the major outcrop faces does not allow us to see these dip changes. Knowledge of the deeper structure enables us to predict where the basin floor should have experienced relative uplift over the active thrust ramp, and to look for evidence of this in the sediment record. And what we see is….. The structure of the basin floor is well defined because the region is well exposed and transected by a set of very deep river valleys. This has enabled us to create a set of well-constrained balanced structural cross sections across the basin, of which the grid shown below is a subset. The nature of the erosion surfaces, internal onlaps , and sediment distribution within the Tertiary basin fill allows the sections to be sequentially restored, enabling the paleostructure to be defined at several stages throughout the life of the foreland basin. The Denjuan channel is anomalous within the section. It sits within an overall basin-floor sheet sand setting in which there is no evidence of other channel systems, and in which we would generally not expect to see them. The channel itself is unusual; it comprises a single deeply erosive surface with a thin channel lag; the remainder of the channel fill comprises sheet sands. More commonly, channel system deposits consist of nested channel complexes with multiple episodes of cut, fill and spill. The model we apply for this channel is that it is a local response to slope disequilibrium generated by a basin floor uplift over a deeper thrust ramp. Alternative depositional system drivers such as variations in sediment supply or sea level changes would not produce this style of channel cut and fill. The Sommet de Denjuan channel Discussion: A similar model of structural control of stratal patterns can also be applied on a larger scale to the entire turbidite sequence. Its base commonly consists of an onlap facies (the Marnes Brunes Inferieures Member); the top of the succession, where preserved, consists of a similar lithofacies (the Marnes Brunes Superieures). We conjecture that this consists of an offlap facies, generated by continued movement over a major (and identifiable) thrust ramp. For discussion of this, please talk with the poster authors! Marnes Brunes Superieures Marnes Brunes Inferieures Gillian Apps and Frank Peel BHP Billiton, 1360 Post Oak Blvd, Houston, TX 77056