Very-fine peloidal to fine-coarse oolitic-bioclastic grainstones with chert and hummocky cross stratification (HCS) interbedded with pelagic lime-mudstones occur in Upper Jurassic successions (Milanos Formation) of the Subbetic (Betic Cordillera, Southern Spain).These strata were deposited in a subsiding trough developed southward of the Southern Iberian Continental Margin, far away from the continental areas.
The grainstone intervals are 10 cm to 5 m thick; horizontal lamination in the lower part upward evolves into HCS and symmetrical wave ripple lamination near the top, being usually capped with wave ripples. Large-scale coarsening-upward cycles from very-fine to fine peloidal grainstones to medium/coarse oolite grainstones have also been recognized in some sectors, locally with thick cross bedded coarse-oolite grainstone units a top. The interbedded lime muds and marls contain ‘filaments’, Saccocoma, sponge spicules and radiolarians.
These deposits were previously interpreted as tempestites (Vera and Molina, 1998) formed from below the storm wave base to very shallow conditions for the top of the cross bedded oolites. New observations suggest these grainstones to be the product of breaking internal waves (IWs) and the associated flows (internalites sensu Pomar et al., 2012) in an environment dominated by pelagic sedimentation. Breaking of internal waves on sloping sea bottom creates episodic high-turbulence events and remobilizes sediments at the depth where the pycnocline intersects the sea floor. The swash run-up produces erosion and the backwash return flow can bypass the breaker and travel downdip where the oscillatory-flow component of the IWs become dominant and form the characteristic HCS bedforms (Morsilli et al., 2012).
The coarse oolitic grainstone with wedge-shaped sets made by gently seaward-dipping parallel lamination, and the sets of low-angle up-slope dipping parallel lamination hold characteristics similar to the ridge-berm-swash zone of modern beaches and are here interpreted to represent the “internal beach”.
This scenario evidences the HCS not be linked to the storm wave base but to the bathymetry of the pycnocline. This alternative interpretation of the Subbetic Jurassic limestones with chert and HCS makes easier to understand their presence in the pelagic zones where the storm and hurricanes wave actioncan be considered “out-of-context”.
Morsilli, M. and Pomar, L. (2012). Terra Nova, 24, 273-282.
Pomar, L., Morsilli, M., Hallock, P. and Bádenas, B. (2012). Earth-Science Reviews, 111, 56-81.
Vera, J.A. and Molina, J.M. (1998). Sedimentary Geology, 119, 103-121.
Topics: Physical sedimentary processes , Topics: Shelf and shallow water sedimentation , Topics: Pelagic sedimentation
