The Calcarenite di Gravina Formation in Matera (southern Italy): New insights for coarse-grained, large-scale, cross-bedded bodies encased in offshore deposits

Excellent outcrops of the upper Pliocene-lower Pleistocene Cal-carenite di Gravina around Matera (southern Italy) provide continuous exposure of coars e-grained, elastic basin-margin, shoreline to offshore facies. Among these facies, the most conspicuous and volumetrically important are the transitio n-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 sedimen t 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 si mple sequences. Parasequences formed during stillstands of sea level and si mple sequences during high-frequency cycles of relative change of sea level . These building blocks are stacked in a backstepping configuration and onl ap 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. Th e reflection patterns of these seismic units resemble the bedding architect ure 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-Pleist ocene accretional units in Matera indicates that they may represent the sam e genetic process: progradation of elastic 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 offsh ore deposits, such as some sandbanks. Differentiating between these lateral ly extensive and seaward-prograding cross-bedded sand bodies and some coars e-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-slop e and prograding shoreface lithosomes. This difficulty may be acute where o nly seismic or well-log data are available; however, certain other architec tural characteristics, such as stacking and preservation of facies belts an d position of bounding surfaces, as well as differences in associated sedim entary structures and fossil content, may be used for interpretation. The Matera example provides a mechanism for emplacement in offshore setting s of elongate and strandline-parallel sand and gravel deposits that prograd e seaward and that preserve a coarsening-upward internal succession. This a rticle offers an interpretation for other ancient examples of large-scale, cross-bedded lithosomes encased in offshore deposits.