Investigating Multiscale Sedimentological Heterogeneity in Upper Cretaceous Carbonate Platforms, Tremp Basin, South-Central Pyrenees

Abstract

During the Upper Cretaceous, rudist-bivalve-dominated carbonate platforms formed within shallow water on the margins of the Tethys Ocean. These are characterised by diverse carbonate facies, often arranged complexly on a sub-seismic scale, and formed by biogenic build-ups, bioclastic shoal bodies and prograding margin clinoforms. Understanding the controls on this heterogeneity is necessary to accurately reconstruct the facies architecture. This thesis is an integrated multi-scale study, defining the geometry, development, and controls on platform architecture and facies distribution within the Upper Cretaceous of the Tremp Basin from platform- to outcrop scale. The five investigated carbonate platforms (Cenomanian to Santonian) are separated by major flooding surfaces, each corresponding to a sea-level oscillation on the scale of ca. 3 My. Correlation of smaller-scale eustatic sea-level variations to changes in platform architecture was not possible, implying that regional processes pose a strong control on platform development on smaller time-scales. High-detail cross-sections of the basin show how the architecture of these platforms changed following a variety of controlling mechanisms. These include a faultcontrolled margin in the early stages and a pelagic drape following the OAE2 at the Cenomanian-Turonian boundary. Subsequent global sea-level fall resulted in subaerial exposure and the deposition of a lowstand wedge, while later global sea-level rise resulted in successive retrogradation of the platform margins, and subsidence and increased sedimentation rates resulted in stepwise thickening of the younger platform successions. Here, local antiform growth also resulted in differential subsidence and laterally uneven thickness development. Comparison of a detailed sequence stratigraphic interpretation of the Santonian Bastus Platform with stratigraphic forward models produced as part of a partner project at Royal Holloway University of London provided insight on possible controlling mechanisms on stratigraphic architecture and facies distribution. The architecture of the Bastus Platform could be modelled using variations in carbonate productivity or via introducing tectonic tilt, while keeping eustatic sea level constant. However, introducing variations in eustatic sea level did not result in fitting models. This demonstrates that changes in carbonate productivity and differential subsidence are possible controls on sequence stratigraphic development of carbonate systems, next to the commonly acknowledged sea-level variations. Geobody dimensions and internal structure, gathered from digital outcrop data, show shoal complexes and infralittoral prograding wedges on the scale of several 10s by few kilometres, with thicknesses of few metres and several 10s of metres, respectively. Biogenic build-ups vary from few 100s metres to few kilometres in both width and length, and show either sigmoidal internal structure or are formed of composite bioherms and biostromes. Platform morphology and architecture, and hydrodynamic and ecological factors are interpreted to pose the underlying controls on geobody shape and size in these examples. Comparison with similar bodies in various Phanerozoic examples shows that unique width, length and thickness relationships may exist within each platform, but are not universal or directly transferrable to similar platforms. The combined results of this study highlight how basin-scale reconstructions are necessary as a framework for understanding the multi-scale variety of controls on platform architecture. On platform scale, knowledge of controls on sequence formation is fundamental for prediction of facies architecture. Only with this information can adequate analogues be defined and comparisons of geobodies between different platforms be made confidently.

Date of Award	1 Aug 2017
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Catherine Hollis (Supervisor) & Stefan Schroeder (Supervisor)