High-resolution sequence stratigraphy in sedimentary discontinuities

Abstract

Abstract Since thirty years, the development and the use of sequence stratigraphy concepts have cause a growing interest for unconformities as bounding surfaces of stratigraphic units. Only a few studies focalized on the comprehension of diagenetic and sedimentary processes, and timing of formation of sedimentary discontinuities. However, these processes represent the only "sedimentary" record of environmental changes, during the formation of the discontinuity. This study based on an integrated approach (field data, thin sections, cathodoluminescence, geochemical analysis), aims to present an example of "virtual" stratigraphic sequence recorded by a sedimentary discontinuity. In the Lodève region (southwestern margin of the French South-East Basin), upper Hettangian to Sinemurian carbonate series, analog to oolitic carbonate reservoir shows composite discontinuity surfaces. One of these discontinuities has been studied in detail, in order to understand its origin. Several stages of formation have been recognized. A transgressive erosion occurred first. Next, development of Skolithos and Diplocraterion burrows, associated with iron mineralizations, could be indicative of a condensation phase corresponding to the subsequent maximum-flooding period. The burrows' sparitic fills may characterize the highstand period following the maximum-flooding period. Next, alteration and corrosion observed on the surface and in the burrows may indicate an emersion related to a regressive phase and the subsequent lowstand period. Finally, micro mud-mounds (formed by pelecypods and corals trapping fine lime sediment) may indicate a new condensation phase, associated with the transgression and the following maximum-flooding period. Thus, this discontinuity records one and a half relative sea-level variation cycles. Implications on the geometry and petrophysical properties of reservoirs are multiple:these particular discontinuities may be laterally correlated with other surfaces in continental settings (paleokarst) or with marine sequences, changing the high-resolution sequence correlations.The lateral variability of these discontinuity surfaces induces diagenetic and petrophysical heterogeneity (porosity, permeability). Introduction Discontinuity surfaces are surfaces in the sedimentary record related to breaks in the sedimentation and represent hiati of various duration [1]. Frequently, only discontinuity surfaces that are characterized by major changes in environmental conditions (drowning surfaces, palaeokarstic surfaces) or a large time gap (missing biozones) are used as stratigraphic markers. However, a large number of minor discontinuities, below the resolution of biostratigraphy may also record evolution of the sedimentary system. This process-oriented study aim to present an example of "composite surface", a descriptive term used for discontinuity surfaces recording meteoric and marine features [2]; [3]. It will be demonstrated that such surfaces may record sedimentary cycles and that their expression may laterally be variable. These two results are key points for high-resolution sequence correlations and study of petrophysical properties for reservoir characterization. Geologic setting The upper Hettangian to Sinemurian carbonate series localized on the Caussenard Shoal (Lodève region) shows a large set of sedimentary discontinuity, ranging from paleosoil or epikarst to hardground in subtidal environments [4], with composite discontinuity surfaces among them. The Caussenard Shoal region is a NE-SW trending uplifted zone along a hercynian fault set (the Cévenne Fault corridor), separating offshore domain of the French South-East Basin (West-Tethysian passive margin) to the South-East from a large epeiric carbonate platform (50 x 100 km), the Grands-Causses basin, to the North-West [5]; [6] (Fig. 1). From Hettangian to Toarcian time, this zone was affected by extensional tectonic movements, in relation to the first episode of opening of the Tethyan Ligurian Ocean. This extensional period was clearly recorded by the development of syn-sedimentary, SW-NE trending, normal faults and rotation of fault blocks within the Liassic sediments [5].