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Polygonal Fault System in the Cretaceous of the Magallanes Basin, southern Chile


Polygonal fault systems (PFS) due to non-tectonic processes are one type of large-scale diagenetic feature common in fine-grained deposits of many basins worldwide. They are thought to represent the combined action of compaction, volume contraction and episodic hydraulic fracturing. Based on the interpretation of high-resolution 3D-seismic data, seismic attribute analysis and systematic mapping, this contribution documents recently recognized and well-developed PFS formed in the Magallanes Basin, southern Chile. Although polygonal faults seem to be widespread in the Basin and occur in Cretaceous to Paleogene strata, the primary study section is the Cretaceous of the Arenal and Intracampos blocks located in the northern part of the Island of Tierra del Fuego. In general, the Cretaceous of the Magallanes Basin is dominated by fine-grained terrigenous sediments. Within this succession, a deformed interval sandwiched between relatively undisturbed and continuous reflections, shows extensional faults with small throws (<50 m) that form polygonal networks in map view. There are three stratigraphic tiers in the deformed interval separated by two relatively undisturbed reflections more likely associated with important stratigraphic surfaces or horizons. The polygons are largely rectangular to pentagonal in areas of low structural dip and range from several tens to more than 2000 m across; they become partial polygons with increase in dip. Faults have no preferred direction, which together with their layer-bound nature, suggest a non-tectonic origin. Major tectonic faults in the study areas show NNW-SSE and NNE-SSW trends. Thought most polygonal faults are normal, several faults exhibit reverse displacements probably linked to the change from extension (passive margin) to compression (foreland basin) in the mid-Cretaceous. Reverse offsets also occur directly above some basement-involved extensional faults. Amplitude anomalies along some polygonal faults and the occurrence in outcrops of sand injectites in age-equivalent strata suggest that PFS may have locally facilitate post-depositional remobilization and clastic injection. Because PFS might control reservoir geometry, production characteristics and fluid flow on a regional scale, an understanding of the timing and distribution of the PFS in the Magallanes Basin may provide important clues about the petroleum migration and compartmentalization history and future well and reservoir management in the area.