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.
Depositional trends in the Valdorbia section (Central Italy) during the Early Jurassic, as revealed by micropaleontology, sedimentology and geochemistry
In the Umbria-Marche basin open marine Jurassic sediments are well exposed in the Valdorbia section (ENE of Gubbio, Central Italy). The time interval considered here is from the Carixian to the Early Aalenian. The depositional units, already dated by means of ammonites and calcareous nannofossils, are: limestones and cherty limestones, Pliensbachian in age («Corniola» = COR); marls of Early - Middle Toarcian age («Marne del Monte Serrone» = MS), including black shales in the Tenuicostatum Zone, reddish nodular calcareous marls and limestones, Middle Toarcian to Early Aalenian in age, which constitute the «Rosso Ammonitico Umbro-Marchigiano» (= RAUM); and bivalve-bearing cherty limestones, Aalenian in age («Calcari a Posidonia» = CP)
Morphology and ecological significance of Zoophycos in deep-sea sediments off NW Africa
Abstract Eighty-six large-diameter (15–50 cm) sediment cores taken off the NW African continental slope have been examined for their bioturbation structures. The Zoophycos “spreiten” — burrows appearing in profile as series of curved arcs — were found to be particularly useful for environmental analysis, because they are (1) widely distributed and numerous in Quaternary sediments, (2) easily identifief due to the characteristic structures in vertical and horizontal core sections, and (3) extraordinarily well preserved. Zoophycos is found off NW Africa in water depths > 2,000 m, in sediments with Corg contents of 0.3–1.8%. It is possible to recognize in the core sections all of the morphological spreiten types of Zoophycos which were described from ancient sediments. All of these spreiten are formed with two modifications classified as “U-type” and “J-type”, according to the shape of the basic tube. The distribution of U- and J-types is significantly related to Corg content and probably depends on the available oxygen content in the respiration water. Morphological analysis leads us to assume that sipunculid animals are the creating organisms. Vertical sediment mixing can be measured by using tracer particles derived from single “event” layers. It is explained by excursions of the creating organism and/or by its active movement of the respiration water. Excellent preservation is a typical feature of Zoophycos, which is evident all over the fossil record. This is due to the considerable depth in the sediment at which the organism producing Zoophycos lives. Core sections with maximum frequency of Zoophycos, indicating optimum living conditions, can be correlated over great distances. For these horizons, it is possible to estimate roughly the maximum population density of Zoophycos-creating organisms, yielding 1 animal/100 m2.