Claude-Alain Hasler

Isotopic, paleontologic, and ichnologic evidence for late Miocene pulses of marine incursions in the central Andes

Recognition of an inferred Miocene marine incursion affecting areas from Colombia through Peru and Bolivia and into Argentina is essential to delineate the South American Seaway. In Bolivia, corresponding strata of inferred marine origin have been assigned to the late Miocene Yecua Formation. We carried out high-resolution δ 13 C and δ 18 O isotopic studies on 135 in situ carbonates from 3 outcrops, combined with detailed sedimentologic, paleontologic, and ichnologic analysis. Four less negative δ 13 C excursion levels were recorded that coincide well with beds containing marine body (barnacle) and trace ( Ophiomorpha ) fossils. These strata are interbedded with red-green beds containing mudcracks, plant roots, gypsum, and trace fossils of the continental Scoyenia ichnofacies. Our data are significant in that they show for the first time four possible short-lived marine incursions in the Bolivian central Andes during the late Miocene. The result is constrained by a new U-Pb date of 7.17 ± 0.34 Ma at the top of Yecua strata.

Discovery of Miocene to early Pleistocene deposits on Mayaguana, Bahamas: Evidence for recent active tectonism on the North American margin

We report here the discovery of Miocene, Pliocene, and early Pleistocene shallow-marine carbonates on Mayaguana Island (southeastern Bahamas) that have so far not been observed on any other Bahamian island. Spanning more than 17 m.y., but <12 m thick, this stratigraphic succession only occurs along the northern coast of the island, indicating that the Mayaguana Bank underwent minor subsidence throughout the late Cenozoic and was tilted toward the south during the Quaternary. In addition to considerably extending the stratigraphic record of the Bahamas Islands, our findings demonstrate that these carbonate banks were at different elevations and subsided at different rates during the Neogene. The young age of the tilting event detected on Mayaguana further shows that parts of the southeastern margin of North America have recently undergone tectonic activity a long way from its actual boundary with the Caribbean plate.

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.

Rule-Based Modelling of Modern-Day and Past Carbonate Shoals Environments

Carbonate reservoirs tend to be particularly heterogeneous because, besides physical sedimentary processes, biological and chemical inputs play an important role in shaping the initial depositional architecture. In addition to these processes, diagenetic overprints, which can either follow or cross cut the primary depositional architecture, introduce another level of complexity by altering the primary depositional porosity and permeability. This combination of multiple processes interacting in both a temporal and spatial sense creates carbonate reservoir characteristics with complex architectures and heterogeneities on various scales, i.e. geobodies. As a result, pattern-based modelling is not always applicable, especially because of the difficulties arising in the identification and definition of discrete geobodies. In order to better predict and model geobodies, research developments are steered toward process-oriented methods for interwell modelling. This extended abstract presents a process-oriented approach using cellular automata applied to interwell-modelling.