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.
TAPHONOMIC TOOLS TO EVALUATE SEDIMENTATION RATES AND STRATIGRAPHIC COMPLETENESS IN ROSSO AMMONITICO FACIES (EPIOCEANIC TETHYAN JURASSIC)
A combined multidisciplinary approach has been applied to calculate minimum values of the stratigraphic completeness and, secondarily, sedimentation rates in 9.2 m thick Rosso Ammonitico facies from central Apennines (Italy) and 11 m thick deposits of the same facies in Southern Spain. Middle - Upper Toarcian expanded sedimentation in Valdorbia section (Umbria-Marche Apennines) and extremely condensed Oxfordian-Tithonian sedimentation at Puerto Escano section (External Subbetic) have been investigated using combined taphonomic, ichnologic and sedimentologic data and analyses. At Valdorbia, infaunal tiering is largely preserved and 27 horizons of infaunal-tiering truncation and casting reveal strong erosional activity forced by tempestite/turbidite events. Therefore, microstratigraphic gaps could be evaluated without biostratigraphic control. In this expanded section, 13 horizons of firm- and hardgrounds have been recorded showing simple or gradational tiering. Conversely, in the condensed Puerto Escano section, taphonomic analysis reveals 25 horizons of bioclasts truncation (mainly in ammonites), and 56 horizons of firm-hardgrounds intensively bioturbated. In Valdorbia rather than in Puerto Escano section, the evaluation of flattening in burrows and spherical bioclasts reveal a measurable mechanical compaction and dissolution. In addition, Rosso Ammonitico at Valdorbia section favoured the calculation of decompaction coefficients (nd) for each lithology easier than in Puerto Escano section. In condensed and essentially hiatal Rosso Ammonitico, mottled deposits due to intense bioturbation dominate and tiering cannot be recognizable. This fact is accentuated by usual overprinting of elementary depositional events, which in turn hampered the accurate calculation of missing deposits. Therefore, in condensed Rosso Ammonitico the latter was only available in terms of minimal missing-record trough the analysis of truncated bioclasts.
Allodapische Kalke, Turbidite in Riff-Nahen Sedimentations-Becken
Summary Allodapic limestones, turbidites in near reef sedimentary basins Allodapic limestones have the following characteristics: ( 1 ) Type of sequence. Interstratification of pelitic layers with clastic limestones. Regular beds which can be traced over long distances without considerable change in thickness. Absence of features indicating shallow water. Decrease, in a given series, of individual and overall limestone thickness in the same direction as grain size decreases (horizontal grading). ( 2 ) Sedimentary features of individual limestone beds (Fig.1). Lower contact sharp, sometimes with flute and groove casts; graded bedding, upper contact mostly indistinct. Larger pebbles near lower contact, sometimes imbricated. Planar bedding becomes gradually thinner towards the top of the bed, grading into lamination. Sometimes current ripple lamination and convolute bedding. Sorting includes fossils and is poor in coarse basal part of bed. Grains are very poorly rounded. ( 3 ) Geometry. Each of the detrital limestone beds forms a lenticular body. The coarser basal layers attain their maximum thicknesses upstream, while the finer grained upper parts have their maxima further downcurrent in the order of grain size (Fig.2). ( 4 ) Bio-facies. The pelitic intercalations are formed in pelagic “Stillwasser”. Both nectonic and planctonic fossils occur statistically scattered, benthos absent. In contrast, detrital limestones consist predominantly of fragments of shallow marine benthos, mostly reef-dwellers, with an admixture of pelagic fossils, which are rare in contemporaneous reef-complexes. Benthonic fossils are never in living position, but sorted according to grain size. Ooids are common. It is concluded that the calcareous detritus was derived from reefs. This material was shed episodically into near by basins through turbidity currents. A model for transportation and sedimentation is presented in order to relate certain sedimentologic features to particular sedimentary processes.