From marginal to axial tidal-strait facies in the Early Pleistocene Siderno Strait

This geological guide presents the description of locations associated with a two-day field trip arranged in relation to the 10th International Congress of Tidal Sedimentology (Tidalites), Matera, Italy. The field guide describes sedimentological features of the largest among a series of tectonically controlled tidal straits that dissected the Calabrian Arc in southern Italy during the Early Pleistocene. The WNW-ESE trending, 50x20 km-wide Siderno Strait connected the Tyrrhenian with the Ionian seas. Due to tidal phase opposition between the two basins, continuous water-mass exchanges occurred through the strait, leading to powerful, bi-directionally flowing tidal currents. Sediments filling the Siderno Strait derived from both fluvial supply from the margins and intra-basinal autochthonous carbonate-factory debris. The main objective of the two-day field trip is to guide the visitor through a cross-section of the ancient strait, starting from one of the margins, ending in the deeper axial zone. The focus during the day one is on strait-margin deltaic fluvial-dominated deposits, shed from the tectonically-controlled, northern border and reworked by tidal currents in their distal reaches (delta front). Erosively-based, 4-5 m-thick pebbly-sandstone strata intercalated with 2-3 m-thick tidally-generated cross strata stack into a ca. 170 m-thick succession, exposed in a series of outcrops progressively located down-current with respect to the inferred entry point to the north. The focus of the day two is a ca. 150-190 m-thick succession consisting of cross-stratified mixed (bioclastic-siliciclastic) deposits, forming a series of WNE-ESE-oriented, elongated ridges that accumulated in the south-eastern axial zone of the Siderno Strait. The selected stops offer panoramic views of exceptionally continuous sections and close-up observations, revealing different scales of depositional architectures and a variety of sedimentary structures and trace fossils that record the development of these tidal sand ridges during the strait lifespan. The interplay between the tectonic uplift of a central bedrock sill and a number of syn-sedimentary faults and high-frequency relative sea-level changes (induced by glacio-eustacy and active tectonics) can be deciphered from the architecture of the tidally-generated cross strata composing the main body of the ridges

Anatomy of a mixed bioclastic–siliciclastic regressive tidal sand ridge: Facies-based case study from the lower Pleistocene Siderno Strait, southern Italy

Sand ridges, a common feature of modern open shelves, reflect persistent currents and sediment availability under recent transgressive conditions. They represent the largest bedforms in the oceans and, as such, can yield information on long-term oceanographic processes. However, there is a limited number of tidal sand ridges documented from the rock record, examples of regressive tidal sand ridges are scarce and studies describing ridges in straits are even more rare. This study analyses a Gelasian succession within a structurally controlled, tide-dominated strait in the Siderno Basin, southern Italy. The strait connected two wider basins, and accumulated sediments reworked by amplified tidal (bi-directional) currents. A series of tidal sand ridges with superimposed dunes developed close to the south-eastern end of the strait, where bathymetry was deeper and flow expansion occurred. One of the best-exposed tidal sand ridges, 65 m thick, crops out along a ca 2 km long cliff. Large-scale, ESE-prograding, seaward-offlapping shingles contain sets of bioclastic–siliciclastic, coarse-grained, cross-stratified sandstones, erosionally overlying upper Pliocene shelf marls and fine-grained sandstones. Cross-strata show angular, tangential and sigmoidal foresets with compound architectures and a SSE migration, i.e. oblique to the main growth direction. Fossil content indicates open-marine conditions. The succession changes abruptly across an erosion surface to non-tidal, highly burrowed mixed siliciclastic–bioclastic fine-grained sandstones, less than 15 m thick. Documented features reflect stages of nucleation, active accretion and abandonment of an individual sand ridge, during a complete cycle of relative sea-level change. The ridge formed during a phase of normal regression, with accretion occurring during an initial highstand and the ensuing falling stage. During the lowstand the ridge was split into several minor bodies by enhanced tidal currents. The ensuing transgression draped the moribund ridge with tabular strata, whereas final highstand shelf sedimentation reworked the top of the underlying sand body with weak currents

Anatomy and origin of authochthonous late Pleistocene forced regression deposits, east Coromandel inner shelf, New Zealand: implications for the development and definition of the regressive systems tract

High-resolution seismic reflection data from the east Coromandel coast, New Zealand, provide details of the sequence stratigraphy beneath an autochthonous, wave dominated inner shelf margin during the late Quaternary (0-140 ka). Since c. 1 Ma, the shelf has experienced limited subsidence and fluvial sediment input, producing a depositional regime characterised by extensive reworking of coastal and shelf sediments during glacio-eustatic sea-level fluctuations. It appears that only one complete fifth-order (c. 100 000 yr) depositional sequence is preserved beneath the inner shelf, the late Pleistocene Waihi Sequence, suggesting any earlier Quaternary sequences were mainly cannibalised into successively younger sequences. The predominantly Holocene-age Whangamata Sequence is also evident in seismic data and modern coastal deposits, and represents an incomplete depositional sequence in its early stages of formation. A prominent aspect of the sequence stratigraphy off parts of the east Coromandel coast is the presence of forced regressive deposits (FRDs) within the regressive systems tract (RST) of the late Pleistocene Waihi Sequence. The FRDs are interpreted to represent regressive barrier-shoreface sands that were sourced from erosion and onshore reworking of underlying Pleistocene sediments during the period of slow falling sea level from isotope stages 5 to 2 (c. 112-18 ka). The RST is volumetrically the most significant depositional component of the Waihi Sequence; the regressive deposits form a 15-20 m thick, sharp-based, tabular seismic unit that downsteps and progrades continuously across the inner shelf. The sequence boundary for the Waihi Sequence is placed at the most prominent, regionally correlative, and chronostratigraphically significant surface, namely an erosional unconformity characterised in many areas by large incised valleys that was generated above the RST. This unconformity is interpreted as a surface of maximum subaerial erosion generated during the last glacial lowstand (c. 18 ka). Although the base of the RST is associated with a prominent regressive surface of erosion, this is not used as the sequence boundary as it is highly diachronous and difficult to identify and correlate where FRDs are not developed. The previous highstand deposits are limited to subaerial barrier deposits preserved behind several modern Holocene barriers along the coast, while the transgressive systems tract is preserved locally as incised-valley fill deposits beneath the regressive surface of erosion at the base of the RST. Many documented late Pleistocene RSTs have been actively sourced from fluvial systems feeding the shelf and building basinward-thickening, often stacked wedges of FRDs, for which the name allochthonous FRDs is suggested. The Waihi Sequence RST is unusual in that it appears to have been sourced predominantly from reworking of underlying shelf sediments, and thus represents an autochthonous FRD. Autochthonous FRDs are also present on the Forster-Tuncurry shelf in southeast Australia, and may be a common feature in other shelf settings with low subsidence and low sediment supply rates, provided shelf gradients are not too steep, and an underlying source of unconsolidated shelf sediments is available to source FRDs. The preservation potential of such autochthonous FRDs in ancient deposits is probably low given that they are likely to be cannibalised during subsequent sea-level falls

Cyclicity in non-marine foreland-basin sedimentary fill: the Messinian Conglomerate-bearing succession of the Venetian Alps (Italy)

Special Publication of the International Association of Sedimentologist

Facies and processes in a Gilbert-delta-filled incised valley (Pliocene of Ventimiglia, NW Italy)

The incised valley of Ventimiglia, located along the Ligurian coast (NW Italy), was cut by deep river erosion during the Messinian sea-level fall and is connected seawards to a slope canyon. During Pliocene, the valley was flooded by the sea and transformed into a coastal embayment or ria. The infill sequence of the incised valley is up to 500 m thick. The paleovalley floor is locally paved by thin remnants of subaerial scree deposits, abruptly overlain by up to 150 m thick bathyal marls, above which a number of stacked prograding conglomerate Gilbert-type deltas constitute most of the valley fill. Gilbert deltas present 15\ub0\u201325\ub0 dipping clinoforms, 50 to 250 m thick, and are alternated with up to 20\u201330 m thick marls intervals. This unusual character of incised-valley-fill sequence, can be accounted for by the rapid and high-amplitude eustatic sealevel rise that followed the Messinian event, and by the progradation occurring on a narrow and steep-gradient shelf, tectonically controlled by the tilting and collapse of the margin. High and coarse sediment supply was provided by the uplifting Alpine chain. A remarkable analogy in facies patterns and depositional setting is observed with the high-latitude Holocene fan-delta systems described by Prior and Bornhold [Prior, D.B., Bornhold, B.D., 1990. The underwater development of Holocene fan deltas. In: Colella, A., Prior, D.B. (Eds.), Coarse-Grained Deltas: Spec. Publs. Int. Ass. Sedim., vol. 10, pp. 75\u201390.] in the fjords of the British Columbia. Both examples are characterized by high rate of sea-level rise after the entrenchment stage, and predominance of massflow processes and debris-avalanching in the first stage of progradation, followed, in the later stages of delta progradation, by deposition of better-organized and stratified foreset beds on the delta slope predominantly by inertia and turbidity flows. A large facies variability is observed in the Gilbert-type delta complex, recording deposition under a wide range of physical conditions, both in individual and successive wedges. Long-term evolution of the valley fill shows a general trend from deep-water to shallow-water deltas and from fluvial-dominated to wave-influenced deltas

Anatomy and evolution of the normal-faulted Lower Pliocene fill of the northern Crotone Basin (southern Italy)

The complex development of the northern Crotone Basin, a forearc basin of the Calabrian Arc (Southern Italy), has been documented by sedimentological, stratigraphic and structural analyses.This Mediterranean-type fault bounded basin consists of small depocentres commonly characterized by a mix of facies that grades from continental to shallow marine.The lower Pliocene in¢ll of the Crotone Basin consists of o¡shore marls (Cavalieri Marl) that grade upwards into a shallow-marine to continentalsuccessionupto850mthick(ZingaFormation).Thesuccessionissubdividedintothree mainstratalunits:Zinga1,Zinga2,Zinga3boundedbymajorunconformities.TheZinga1stratalunit grades from the Cavalieri Marl to deltaic and shoreface deposits, the latter organized into several stacked progradational wedges that show spectacular thickness changes and progressive unconformities related to salt-cored NE-trending growth folds and listric normal faults.The Zinga 2 stratal unit records a progressive and moderate deepening of the area, marked by £uvial sedimentation at the base, followed by lagoonal deposits and by a stacking of mixed bioclastic and siliciclastic shoreface units, organized into metre-scale high-frequency cycles. Deposition was controlled by NE- trending synsedimentary normal faults that dissected the basin into a series of half-grabens. Hangingwall stratigraphic expansion was compensated by footwall condensed sedimentation. The extensional tectonic regime continued during sedimentation of the Zinga 3 stratal unit. Deposition con¢ned within structural lows during a generalized transgressive phase led to local enhancement of tidal £ows and development of sand-wave trains.The tectonic setting testi¢es the generalized structural domain of a forearc region.The angular unconformity at the top of the Zinga 3 stratal unit is regional, and marks the activation of a large- scale tectonic phase linked to strike- slip movements

Tidal depositional systems in the rock record: a review and new insights

Some of the principles of tidal-wave theory and examples of mega-, macro-, meso- and microtidal coasts are reviewed, as well as sedimentary successions showing general tidal signals (tidalites) and thinly-laminated, cyclically stacked tidal strata (tidal rhythmites). Although tidalites are well known for their muddy stratification, some of the most spectacular tidal deposits are the sand-rich, cross stratified successions that accumulated as tidal dunes, compound dunes and tidal bars in deltas, estuaries, shelves and straits. Recent progress has been made on modelling of ancient tidal strata, (1) in relation to sea-level rise and fall, (2) in recognition of the systematic changes occurring within the important fluvial-marine transition zone, (3) in the prediction of ancient tidally influenced deposits using shoreline morphology, shelf width and accommodation to supply ratios, and in (4) generation of palaeo-ocean models and the computation of tidal dynamics in ancient seas and seaways. Recent key insights into ancient tidal strata include the recognition of fluid-mud deposits, the realization of the significance of tidal bars versus tidal dunes, the use of palaeogeographic data for prediction of tidal sediments and the recognition of ancient tidal-strait deposits. © 2012 Elsevier B.V.Sergio G. Longhitano, Donatella Mellere, Ronald J. Steel, R. Bruce Ainswort