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1 Departament de Ciències de la Terra, Universitat de les Illes Balears, 07071 Palma de Mallorca, Spain; lpomar{at}uib.es
2 Dipartimento di Scienze Geologiche, Università della Basilicata, 85100 Potenza, Italy; tropeano{at}unibas.it
Luis Pomar is a professor of geology at the University of the Balearic Islands on Mallorca, Spain. He received the M.S. degree and Ph.D. in geology from Barcelona University. His research is concentrated on carbonate sedimentology and high-resolution sequence stratigraphy. Luis's special interest is the process-product relationships in carbonate systems. Currently he is doing outcrop-oriented studies on the islands of Menorca and Mallorca and in southern Italy.Marcello Tropeano is a researcher at the University of Basilicata, Italy. He received his Ph.D. in earth science in 1994 from the University of Bari, Italy. His research interests deal with stratigraphy and sedimentology, particularly of southern Apennines foreland and foredeep successions.
Excellent outcrops of the upper Pliocene-lower Pleistocene Calcarenite di Gravina around Matera (southern Italy) provide continuous exposure of coarse-grained, clastic basin-margin, shoreline to offshore facies. Among these facies, the most conspicuous and volumetrically important are the transition-slope deposits that form large-scale, high-angle, cross-bedded lithosomes. These are laterally extensive, parallel with the paleoshoreline, and show seaward progradation. We interpret them to represent avalanches of sediment swept out onto a depositional slope, below wave base, from the shoreface zone by storm waves and wind-driven currents.
Three types of building blocks are recognized based on bedding patterns and facies architecture: embryonic parasequences, mature parasequences, and simple sequences. Parasequences formed during stillstands of sea level and simple sequences during high-frequency cycles of relative change of sea level. These building blocks are stacked in a backstepping configuration and onlap onto Cretaceous limestone substrate. Backstepping is believed to be due to a tectonically forced transgression that is punctuated by high-frequency cycles of sea level.
Modern analogs for these building blocks are the Holocene prograding prisms detected in high-resolution seismic lines of the Mediterranean shelves. The reflection patterns of these seismic units resemble the bedding architecture of the Matera simple sequences and parasequences and show similarities of shape, size, position, and orientation of coastal setting, and direction of progradation. Comparison of the Holocene prisms and the Pliocene-Pleistocene accretional units in Matera indicates that they may represent the same genetic process: progradation of clastic prisms below the wave base level.
The Matera accretional units also show similarities with other examples of laterally extensive, large-scale, cross-bedded sand bodies encased in offshore deposits, such as some sandbanks. Differentiating between these laterally extensive and seaward-prograding cross-bedded sand bodies and some coarse-grained Gilbert-type deltas is difficult, however, if interpretations are based only on two-dimensional (2-D) outcrops in dip section. Without high-resolution data it is also difficult to distinguish between transition-slope and prograding shoreface lithosomes. This difficulty may be acute where only seismic or well-log data are available; however, certain other architectural characteristics, such as stacking and preservation of facies belts and position of bounding surfaces, as well as differences in associated sedimentary structures and fossil content, may be used for interpretation.
The Matera example provides a mechanism for emplacement in offshore settings of elongate and strandline-parallel sand and gravel deposits that prograde seaward and that preserve a coarsening-upward internal succession. This article offers an interpretation for other ancient examples of large-scale, cross-bedded lithosomes encased in offshore deposits.
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