Storm deposits and storm‐generated coarse carbonate breccias on a pelagic outer shelf (South‐East Basin, France)

Abstract

Uppermost Jurassic limestones of the South‐East Basin (France) are organized into four facies associations that were deposited in four distinct zones: (1) peritidal lagoonal limestones; (2) bioclastic and reefal limestones; (3) pelagic lime mudstones; (4) lime mudstones/calcarenites/coarse breccias. Calcarenite deposits of zone 4 exhibit sedimentary structures that are diagnostic of deposition under wave‐induced combined flow. In subzone 4a, both vertical and lateral transitions from lime mudstone/calcarenite to breccia indicate in situ brecciation under wave‐cyclic loading. Breccias were produced by heterogeneous liquefaction of material previously deposited on the sea floor. Deposits in subzone 4a record relatively long periods (>400 kyr) of sedimentation below wave base, alternating with periods of deposition under wave‐induced currents and periods of in situ deformation. In this zone, storm waves were attenuated by wave–sediment interaction, and wave energy was absorbed by the deformation of soft sediment. With reference to present‐day wave attenuation, water depths in this zone ranged between 50 and 80 m. Landwards of the attenuation zone, in zone 3, storm waves were reduced to fair‐weather wave heights. Storm wave base was not horizontal and became shallower landwards. As a consequence, water depth and wave energy were not linearly related. On a small area of the seaward edge of subzone 4a, cobbles were removed by traction currents and redeposited in subzone 4b. There, they formed a 100‐m‐thick wedge, which prograded over 3 km and was built up by the stacking of 5‐ to 20‐m‐thick cross‐stratified sets of coarse breccia. This wedge records the transport and redeposition of cobbles by a high‐velocity unidirectional component of a combined flow. The increase in flow velocity in a restricted area is proposed to result from flow concentration in a channel‐like structure of the downwelling in the gulf formed by the basin. In more distal subzone 4c, the hydrodynamic effect of wave‐induced currents was quasi‐permanent, and brecciation by wave–sediment interaction occurred only episodically. This indicates that, seawards of the attenuation zone, hydrodynamic storm wave base was deeper than mechanical storm wave base. Uppermost Jurassic carbonates were deposited and soft‐sediment deformed on a hurricane‐dominated ramp of very gentle slope and characterized by a zone of storm wave degeneration, located seawards of a zone of sedimentation below wave base.

Source and depositional processes of coarse-grained limestone event beds in Frasnian slope deposits (Kostomłoty-Mogiłki quarry, Holy Cross Mountains, Poland)

“…The scourand-fill structures observed at the bases must have been created by storm-induced bottom currents or combined flows winnowing the sea floor (cf. Seguret et al, 2001). The geopetal infillings in fossils suggest rapid burial during storm events (Flügel, 2004, p.178).…”

Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia

“…The process of cyclic wave loading invoked by Seguret et al (2001) may be partly invoked to explain some of the features observed in the Hoogland breccias. Cyclic wave loading of the sea-floor entails the periodic displacement of sub-surface sediments (Suhayda, 1977), such that slowly applied shear stresses strain the strata and liquefy the muds as pore pressures progressively increase (Clukey et al 1985).…”

“…As the storm swell dissipated, the continued application of layer-parallel shear stresses macerated any stratigraphy not already cemented to form the platy, intraclastic matrix. The newly produced slurry of platy intraclasts and larger rafts would have locked into place once the layer-parallel shear stresses decreased below the Bingham threshold for deformation (Seguret et al 2001). A further decrease in current velocities and shear stresses resulted in the deposition of hummocky crossstratified beds, overlain by wave-oscillation ripples observed at the tops of brecciated intervals.…”

Some key Lower Cretaceous sites in Drôme (SE France)

“…(FERRY et al, 2015). The finding of HCS-like features in some beds has been at the origin of a recent controversy (BOUCHET-TE et al, 2001;SEGURET et al, 2001) about the mechanisms of emplacement of the breccias and therefore the actual depth of the basin (see discussion in FERRY et al, 2015). Fig.…”