Shallowing-upward cycles in pelagic troughs (Upper Jurassic, Subbetic, Southern Spain)
Abstract Abundant calcareous tempestites with chert and hummocky cross-stratification (HCS) occur in Upper Jurassic successions (Milanos Formation) of the Subbetic (Betic Cordillera, Southern Spain). These calcareous tempestite beds are calcisiltites and very fine calcarenites with peloids and bioclasts, and have thicknesses between 25 and 75 cm. Beds show symmetrical wave ripple lamination near the top, and are usually capped with wave ripples. According to palaeogeographic reconstructions, these strata were deposited in a subsiding trough in the pelagic domain of the Southern Iberian Continental Margin, which was the northwestern-most border of the Tethys. Calcisiltite and calcarenite beds deposited in this pelagic trough form small-scale to large-scale shallowing-upward cycles. Shallowing-upward cycles with calcareous tempestites in the lower part and oolitic limestone beds in the upper part, have also been recognized in some sectors of the Subbetic pelagic basin. The large-scale cycle occurs in the upper part of the Jarropa Radiolarite Formation, which directly underlies upper Oxfordian, Kimmeridgian and Tithonian strata of the Milanos Formation. Two types of large-scale shallowing-upward cycles have been recognized, according to the presence or absence of thick units of oolitic limestone with large-scale cross-stratification. The water depth during deposition ranged from below, but near, storm wave base at the base of the large-scale cycles to very shallow carbonate platform at the top.
Storm-generated depositional types and associated trace fossils in Lower Carboniferous shallow-marine carbonates of Three Cliffs Bay and Ogmore-by-Sea, South Wales
Abstract Four distinct storm-generated or induced depositional types have been recorded in lower Carboniferous shallow marine limestones of Ogmore-by-Sea and Three Cliffs Bay, South Wales. Type A deposits have whole and oriented horn coral lag deposits and graded limestones with large regular Zoophycos . They are interpreted as storm-generated turbidite deposits in an area below storm wave base. Type B deposits are characterized by graded limestones and short wavelength hummocky cross-bedding with Rhizocorallium and small arcuate Zoophycos , small Planolites -like burrows and were probably formed by storm currents at depths just above storm wave base. Type C deposits consist of crinoidal lag deposits and long wavelength hummocky cross-bedding with escape burrows and were probably deposited just below fair weather wave base. Type D deposits contain skeletal sandstone lag deposits and large-scale, low-angle cross-bedding, generally without bioturbation (apart from borings), indicating a shore-face or beach environment. The depositional types belong to the Courceyan limestone ( Zaphrentis Zone) and Arundian limestone ( Caninia Zone). It appears that the pattern of trace fossils and bioturbation, as well as sedimentary structure and texture, change characteristically with changing wave effect depth in this area.
Paleontological Studies on Turbidite Sedimentation and Erosiona
The postdepositional sole trails of Flysch psammites occur only in thinner beds up to a thickness particular to each species. This proves instantaneous deposition of the individual beds, as postulated by the turbidity-current theory. The majority of the sole trails are predepositional mud burrows washed out and sand cast by turbidity currents. Thus erosion of an unusual type must have preceded every turbidite sedimentation.