Syndepositional deformation in steep-walled carbonate margins : insights from outcrop and numerical modeling of carbonate platforms in the recent and ancient rock record

Nolting, Andrea
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Syndepositional deformation is common in steep-walled carbonate platforms and is typically manifested as large, open-mode fractures and normal faults. Despite the recognition of syndepositional features and their importance in steep-walled carbonate platform systems worldwide, the controls behind the development and the distribution of early-formed deformation are still poorly understood. There remains a gap in knowledge with regards to the relationships between mechanical properties of carbonate rocks and facies type, age and early diagenesis, which hinders our ability to systematically test and evaluate potentials controls on the development of early deformation. This work investigates (1) how facies type, depositional setting, diagenetic alteration, and age affect rock strength in Pleistocene carbonate rocks; (2) how carbonate platform geometry impacts the development of early deformation; and (3) the control that progradation to aggradation (P/A) ratio and carbonate rock property heterogeneities has on the development of syndepositional deformation. This research utilizes a combination of outcrop-based work and numerical modeling of steep-walled carbonate platforms to aid in identifying and evaluating the controls on the development of early-formed deformation. Mechanical rock properties tied to key facies, depositional setting, age, and diagenetic alteration were characterized from field measurements and laboratory analysis on samples collected from the Island of West Caicos on the Turks and Caicos platform,. Results suggest that rock strength in unburied Pleistocene carbonate rocks is controlled by cement percentage and, to a lesser extent, facies type, where reef facies are stronger than grain dominated facies. Increases in cementation tied to subaerial exposure and calichification is strongly tied to increases in unconfined compressive strength (UCS). Our observations on West Caicos are best explained by periods of long repeated subaerial exposure (and ensuing cementation from early meteoric diagenesis) and brief marine inundation consistent with the climatic conditions of the Pleistocene Epoch, when high-frequency, high-amplitude sea-level oscillations occurred. The observations and rock properties collected on West Caicos were used to populate the material database within the numerical models, allowing for realistic simulation of syndepositional deformation. Numerical models were constructed using ELFEN®, a finite element modeling program that allows for the development of discrete fracture and fault development. Our numerical modeling results suggest that platform geometry, specifically the presence of a high-relief vertical reef wall, and changes in P/A ratio are primarily controls on the development of early-formed deformation. To a lesser extent, facies partitioning and juxtaposition control the intensity, distribution and propagation of deformation. The development of syndepositional deformation in steep walled carbonate platforms is largely a byproduct of the lack of a confining stress in the seaward direction. This leads to the development of a tensile stress state that is prone to failure by open-mode fractures and faults. These deformation features form under the sole application of gravity, in the absence of differential compaction of basinal sediments or external perturbations (e.g. regional tectonics, active faults, etc.), highlighting the syndepositional origin of deformation. Results demonstrate that carbonate platforms that have a vertical to near vertical reef wall and steep angle slopes are routinely modified by syndepositional deformation. These parameters are thus primary controls on platform architecture, stratal geometries through time, and development of preferred failure and fluid flow pathways.