Halokinetic sequences in carbonate systems : an example from the Middle Albian Bakio Breccias Formation (Basque Country, Spain)


In diapir flanks, unconformity-bounded sedimentary packages associated with gravity-driven deposits, controlled by the ratio between the rates of sediment accumulation and diapir growth can be interpreted in the context of halokinetic sequences. The Bakio Breccias Formation (Basque Country, Spain) corresponds to redeposited carbonate deposit that developed in response to the Bakio diapir growth during the Middle Albian. These deposits provide on of the rare documented example of carbonate-dominated halokinetic sequences. The Bakio Breccias Formation consists of an alternation of clast- and matrix-supported breccias, calcirudite, calcarenite and marl, deposited along the flanks of the diapir. The description and the analysis of the Bakio Breccias Formation lead to a new model for carbonate-dominated halokinetic sequences. These sequences differ from their siliciclastic counterpart because sediment accumulation rate is controlled by carbonate platform growth on the topographic relief top of the diapirs, while sediments are preferentially deposited in the mini basins adjacent of the diapirs, in siliciclastic settings. During transgressive system tract, carbonate platform are able to keep up with the sea level rise and to aggrade on top of the diapirs, forming thick and resistant roof, which is assumed to limit the diapir growth and thus to favour the development of halokinetic sequences with low angle unconformities (wedge halokinetic sequences). During late highstand system tract deposition (and lowstand system tract if present), platform progradation results in high sediment accumulation in the adjacent depocenters, loading the autochthonous salt layer and promote diapir growth and creation of topographic relief. In addition, if the diapir roof reaches emersion, karstification of the carbonate platform top may also favour roof destruction and diapir growth. Depending on the thickness of the roof developed previously and the amplitude of the sea level fall, the halokinetic sequences with the emersion and the karstification of the carbonate platform may display high angle unconformities (hook halokinetic sequences). Furthermore, gravity-driven deposits are assumed to be more common in carbonate-dominated halokinetic sequences, compared to their siliciclastic counterparts, since carbonate platform aggradation creates steep slopes on the diapir margins, leading to the partial collapse of the margin, even when limited diapir growth occurs. The carbonate-dominated halokinetic sequence model proposed here is an important tool for the prediction of potential reservoir distribution, seal and hydrocarbon migration in flanks of salt diapirs where carbonate platform developed

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