Geometry and petrophysical parameters of a calcarenitic turbidite lobe (Barremian-Aptian, Pas-de-la-Cluse, France)

Abstract

Several bioclastic resedimentation systems, late Barremian to early Aptian in age, outcrop in southeastern France. The upslope portion of a calcarenitic turbidite lobe outcrops as a cliff, 2.3 km wide and up to 45 m high, oriented roughly perpendicular to the direction of palaeoflow. It has been studied through 31 sections. The studied body is point-sourced and corresponds to a channel/lobe transition located at the toe-of-slope of a Urgonian carbonate platform on the western edge of the Vocontian Trough (French Subalpine Chains). The internal geometry of the lobe is the result of repeated erosion and filling of channels, creating well-defined stacked units. Filling phases result in lobe-shaped morphology. Units evolve from base to top in an ordered way. First, the sediments exported from the platform and deposited in the basin are represented by decimetre-thick, sheet-like and fine-grained Bouma a-e and ab-e sequences. In this first unit, erosion is limited and represented by shallow and symmetrical channels. This initial phase of deposition corresponds to the formation of a lobe with a positive morphology. The next phase of system development is more erosional and this is dominated by migrating, sinuous channels several metres deep filled with coarse-grained, poorly graded turbidites. These channels are first filled through lateral accretion and then sediment spills over to create a sub-lobe. The mixture of channel and lobe features could be considered as a carbonate equivalent of the channel–levee system of siliciclastic deposits. Finally, the system ends with the deposition of a flat-based unit. This unit extends for more than 2 km and is composed of the southwards lateral accretion of hundreds of metres wide sequences each composed of a debris-flow deposit capped by a coarse-grained calcarenite bed. These sigmoidal bodies could be the ultimate forms of a spreading out phenomenon of the lobe. Overall, the Pas-de-la-Cluse system is a coarsening-upward body. As the grain-size is related to the nature of the material, this coarsening-upward implies either an evolution in the nature of the carbonate material produced on the platform (from grainy to bioconstructed), a displacement of the source sediment (from inner platform to margin) or a displacement of the channel head through time (from distal to proximal). The upward evolution from classic turbidites to debris-flow deposits can be explained either by the way the platform facies evolved with time in a depositional sequence or through retrogressive erosion by successive slope failures that cut deeper and deeper into the platform slope. Although its present-day porosity is low, the size of the system and the fact that it is encased in marls make it an interesting outcrop analogue for hydrocarbon exploration.