Evolution of Neogene contractional growth structures, southern Gulf of Mexico

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Aranda García, Mario

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Structural analysis of four contractional growth structures in the Southern Neogene Belt of Mexico was undertaken to test conceptual models of folding mechanisms for contractional deformation and to obtain structural interpretations of important oil prospects and one oil field. Based on seismic data, application of these models provides coherent explanations of structural development incorporating the geometry of the structures as well as the geometry and age of the growth strata, as shown in viable balanced cross-sections, and demonstrate a complex evolution of folding mechanisms in a 3D framework. Folding and faulting are important mechanisms of deformation in the brittle upper crust, and in particular are the main processes of fold-thrust belt development. Excellent examples for the geometries developed in these belts have been documented around the world. However most of them lack growth strata, due mainly to their deep levels of erosion, and for this reason their development between undeformed and final geometry is weakly supported with data. Growth strata are those units deposited during deformation, and ideally they record the entire evolution of a contractional terrain. In modem thrust belts preservation of growth strata is uncommon; marine growth sediments have better potential for preservation of internal and external features than do continental growth sediments. Some excellent examples of contractional growth structures from the southern Gulf of Mexico are used here to test fault-related folding and buckle folding models and to obtain detailed interpretations of one discovered oil field and some important prospects for oil exploration. The structures analyzed are part of the Southern Neogene Belt of Mexico a thrust belt that represents upper crustal shortening in the southern portion of the North America Plate. The Sardinero, Jujo, Tunich and Catemaco anticlines are located in the Veracruz and Campeche-Tabasco-Chiapas basins. These basins were dominantly filled by upper Eocene-Neogene marine deposits. The regional trend of the structures is NW-SE but this orientation is modified in part where the Jurassic salt is present. The basal detachments for the Sardinero and Catemaco structures are located in Paleogene mudstone units and the detachment for the Tunich and Jujo structures is in Jurassic salt. The structures analyzed were selected because they are good examples of contractional growth structures with good seismic data sets and chronostratigraphic well control. The pre-kinematic strata of Sardinero anticline have geometric similarities with a fault-bend fold structure, but growth strata on the backlimb show a more complex evolution. The growth strata were deposited during an early buckle-folding stage between 15.5-3.5 Ma which was-followed by the development of and translation over a fault ramp after 3.5 Ma. The Jujo anticline prekinematic strata has a good fit with predictions for a fault-bend fold and the geometries of growth strata agree with the model. Structural timing from growth strata indicate folding during the mid-Miocene and Pliocene. The Tunich and Catemaco anticlines correspond to curved detachment folds. They have apparent fault-related folding geometries at pre-kinematic levels but they were initiated by buckle folding as the geometries of their growth strata demonstrate. The age of deformation for both anticlines was between 21 to 2.4 Ma and 16.5 to 5.5 Ma respectively.