Storms in the deep: Tempestite- and beach-like deposits in pelagic sequences (Jurassic, Subbetic, South of Spain)

Abstract

Abstract Fine-peloidal- to coarse oolitic-bioclastic grainstones with hummocky cross stratification (HCS) occur interbedded in Jurassic pelagic lime-mudstone successions (Subbetic, Betic ranges, Southern Spain). These strata were deposited in pelagic troughs and swells, away from continental areas, in the Southern Iberian Paleomargin of the Western Tethys. Selected examples presented in this study include: a) coarse oolitic grainstones, encased in pelagic marls, holding characteristics similar to the ridge-berm-swash zone of modern beaches; b) peloid grainstones with HCS interbedded with radiolarite marls deposited on the flanks of volcanic guyots. The interbedded lime muds and marls contain “filaments”, sponge spicules and radiolarians; c) peloid-bioclastic (radiolarians, “filaments”, etc.) grainstone beds with HCS, interbedded with pelagic lime muds; d) crossbedded peloidal-skeletal (Saccocoma) grainstones with HCS and wave ripples on top, interbedded with pelagic mudstones and wackestones with abundant bioturbation and ammonites (Ammonitico Rosso facies). All the examples here presented share: 1) grainstone beds are interbedded with pelagic mudstones and marls; 2) grainstone components were reworked by oscillatory flows superimposed to unidirectional tractive flows (unidirectional ripple lamination and HCS); 3) components, either derived from shallow-water environments (e.g., ooids) or produced in pelagic conditions (e.g., radiolarians, Saccocoma, “filaments”, peloids, etc) were reworked by high-energy processes unrelated to surface storm waves; 4) Internal waves (IWs) propagating along a pycnocline and breaking against a sloping surface are the best candidates to induce the sedimentary structures and sediment organization that characterize these grainstone beds. Coarser sediments “trapped” at the breaker zone form sediment accumulations similar to the sediments caught by the “littoral fence” in the surface beach. This scenario evidences the HCS not to be necessarily linked to the surface storms but to the pycnocline bathymetry, solving the problem of having HCS in pelagic zones where the storm and hurricanes wave action can be considered “out-of-context”.