Grégory Frebourg

Depositional controls on sediment body architecture in the Eagle Ford/Boquillas system: Insights from outcrops in west Texas, United States

A detailed sedimentological study of the middle Eagle Ford/Boquillas (outcrop analog of subsurface producing strata) units was conducted on numerous roads cut along US Highway 90 in Val Verde County and on outcrops in Big Bend National Park, Brewster County, Texas, using field-based petrographical and high-resolution image capture methods (light detection and ranging and GigaPans). This study demonstrates that vertical and lateral facies distribution is controlled by the interaction of sediment productivity under the influence of bottom currents below storm wave base. Vertical cyclicity is the result of alternating periods of lower primary productivity and relatively low sediment accumulation rates (globigerinid argillaceous wackestones) and shorter periods of high primary productivity and higher accumulation rates (pelagic grainstones), driven by the absence or presence of iron from volcanic ash deposition. Lateral variations are controlled by the deposition and reworking of pelagic sediment under the influence of below–storm wave base bottom currents. Pelagic grainstones accumulated in the form of isolated barchanoid hydraulic dunes, sand ridges, coalesced sand ridges, and sand sheets and less commonly as continuous beds. Detailed measurements show that pelagic grainstones have no more than 50% continuity, and ash beds have 72% continuity. Application of sequence stratigraphic principles needs to be done with caution because the deeper-water depositional setting is affected not by sediment input from a shallow-water benthic carbonate factory but by pelagic sediment from the open-marine environment subject to bottom current reworking. Packages of strata may be more reliable for longer-distance correlations.

Anatomy, internal heterogeneities, and early fracture network of a Pleistocene carbonate coastal dune (Rejiche Formation, southeastern Tunisia)

Although eolian deposits are known to record the dominant winds, secondary conditions such as wind reversals during wintertime can also be observed in the petrographic composition and facies succession. Thus, eolian deposits are used here as a local paleoclimatic proxy. The spatial distribution of the depositional facies, early diagenetic imprints, and early fracture network of a coastal Pleistocene eolian ridge in southeastern Tunisia is described using a small-scale GIS model. Facies analysis indicates that coastal dune systems record seasonal cycles. The fracture density and directions are strongly influenced by the depositional facies type. Laminated facies present a higher fracture density compared to more homogeneous facies and show only one major fracture direction, while the more homogeneous facies display a bimodal distribution. Such a difference between these two groups is explained by the heterogeneous distribution of the early calcite cement within the laminated facies. No tectonic activity or overlying strata have affected the Pleistocene dunes under study. Therefore, the mechanism responsible for the fractures could only be related to the own weight of the eolianite and to its internal or underlying lithologic heterogeneity.

The carbonate-evaporite lagoon of Al Dakhirah (Qatar): an example of a modern depositional model controlled by longshore transport

Abstract The southern coast of the Arabian Gulf is considered a good example of an arid carbonate depositional system. The objective of this field investigation is to better elucidate the dynamic controls on the character and pattern of the depositional belts in a carbonate-evaporite lagoonal setting. The interpretation of this lagoon is based on combined field examination and laboratory analyses of surface samples, and this has resulted in an updated depositional model for carbonate-evaporite lagoonal settings. This study highlights the effects of sediment transport by longshore currents, with the formation of sand spits that gradually closed the lagoon during their southwards migration. The associated narrow and elongated back-barrier settings consist of tidal-dominated muddy sediments affected by a network of tidal channels. This wave-dominated high-energy system has generated several carbonate barrier and back-barrier units that migrated southwards and seawards during the last few thousands of years. This updated depositional model significantly differs from shoreline-parallel facies tracts of the classical Trucial Coast model, where the dominant wind is orientated perpendicular to the coastline. Conversely to the sediment belts of the Trucial Coast, the Al Dakhirah lagoon displays a strongly asymmetrical pattern of the sediment belts, marked by a southwards and seawards migration.

The Khuff Formation in the Middle East: New Insight into Regional Stratigraphy and Palaeoenvironmental Reconstruction using Bio-assemblages and Facies Analysis

A multidisciplinary synthesis of outcrop and subsurface data of the carbonates and evaporites of the Late Permian Khuff Formation was carried out in order to constrain the spatial and stratigraphic distribution of the depositional facies.