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Vilomara Composite Sequence numerical modelling

Abstract

Vilomara composite sequence (VCS) was deposited during the regressive part of the transgressive-regressive megasequence of the Sant Llorenç del Munt fan-delta complex (SLM). The VCS comprises an alternation of detrital sediments interbedded with subordinate carbonate platform deposits. The facies associations in the VCS are from proximal to distal: proximal alluvial fan, distal alluvial fan, fan-delta front, carbonate platform, and fan-delta slope and prodelta.

The VCS is structured into seven repetitive, metre-scale transgressive-regressive fundamental sequences. These sequences are bounded by maximum regressive surfaces, which correspond to the time of regressive- to-transgressive turn around of the shoreline, located at the first indication of upward deepening following a regression. These sequences are made up of a transgressive systems tract (TST), overlain by a regressive systems tract (RST), separated by a maximum flooding surface (MFS). They record fluctuations in detrital input, most probably related to a combination of allocyclic (periodic to episodic, relative sea-level changes, tectonic pulses and Milankovitch climatic oscillations) and autocyclic factors (variations in the organization of distributary channels). TSTs develop during periods of low terrigenous input and/or increasing accommodation; RSTs are a response to episodes of high terrigenous input and/or decreasing accommodation space.

From the previous geological settings, the architectural organization has been interpreted as controlled by fluctuations in the sediment supply rate and changes in accommodation. In this contribution, the SIMSAFADIM-CLASTIC program (a process-based forward numerical modelling program created to model sedimentary basins), is used to model the VCS. It models clastic transport and sedimentation, including carbonate production. The program obtains a 3D output that allows to study the geometry and the interrelationships between the different parameters modelled.

The results obtained using SIMSAFADIM-CLASTIC can help to discuss which of both parameters, fluctuations in the sediment supply rate and/or changes in accommodation, has great influence on the geometry and on the facies distribution. From field data, 3 different models have been done taking into account the affection of three different parameters: sediment supply (model A); sea level change (model B); and the combination of both (model C).

From these models, model C is the most accurate model and has a direct correlation with the data obtained in the field. However, model A shows similar structure, despite thickness and some distributions differs, and model C is really different, concluding that the sediment supply play a more important role than accommodation space in the observed sedimentary architecture.