1,721,111 research outputs found
Internal waves vs. surface storm waves: a review on the origin of hummocky cross-stratification
No full textHummocky cross-stratification (HCS) is considered a diagnostic structure of surface storm activity at the shoreface–offshore transition. However, the origin of HCS is still debated. Laboratory experiments have not yet reproduced it and direct observations on the continental shelves do not exist. Most hydrodynamic interpretations invoke pure oscillatory flows, unidirectional-dominated combined flows and oscillatory-dominated flows, but they all share the assumption of HCS to reflect the combined action of surface storm waves and related currents. Within this context of uncertainties, internal waves (gravity waves propagating along the pycnocline) provide an alternative mechanism to explain the origin of HCS. Internal waves breaking on the shelf create episodic high-turbulence events and induce upslope- and downslope currents as well as oscillatory flows at the depth where the pycnocline intersects the sea floor. In this scenario, both the oscillatory- and the unidirectional components needed for HCS to form are not necessarily linked to surface storm waves, but can occur at various depth as far and near there is a pycnocline where internal waves can for
The Calcarenite di Gravina Formation in Matera (Southern Italy): new insights for coarse-grained, large-scale, crossbedded bodies encased in offshore deposits
No full textExcellent 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
Rhodolith assemblages from the lower Tortonian carbonate ramp of Menorca (Spain): Environmental and paleoclimatic implications
No full textLower Tortonian distally steepened carbonate ramp of Menorca mostly consists of foramol and rhodalgal facies deposited in inner-middle ramp, ramp slope and outer ramp settings. Red algae are abundant from the middle ramp to the lower part of the slope and their taxonomic assemblages are clearly related to the bathymetric position. Melobesioids percentage increases basinward, passing from 55.8% in the middle ramp to 97% in the toe of slope. Mastophoroids are more abundant in the middle ramp (43.1%) and decrease toward deeper paleoenvironments (1.2%). Lithophylloids and sporolithaceans appear as accessory components from the middle ramp to slope settings. The percentage of melobesioids and mastophoroids observed in the middle ramp suggest that growth of the rhodoliths started in a water depth below 10–20 m. The deepest occurrence of the rhodoliths is in the ramp slope environment, where the dominance of melobesioids and the low percentage of shallower-water subfamily suggests a water depth range of 70 to 100 m. Shape and structure of rhodoliths are indicative of high-energy conditions in all ramp settings and they do not reflect a decrease in hydrodynamic energy related to water depth increase. These conditions are related to the presence of unidirectional currents that produced cross-bedded grainstones existing in the middle ramp, ramp slope and at the toe of the ramp slope settings. The high percentage of mastophoroids in the shallowest environments and the presence of Lithoporella and Sporolithon in the coralline assemblages suggests that carbonate production took place in tropical waters
Architectural complexity of a carbonate transgressive systems tract induced by basement physiography
No full textThree-dimensional exposure of the Plio-Pleistocene Calcarenite di Gravina Formation around Matera Horst, Italy, permits analysis of the architecture and internal complexities of carbonate sedimentary bodies around a palaeoisland. Spatial organization of the different lithosomes, along with their skeletal composition, sedimentary structures and bedding patterns, helps decipher the key factors that controlled platform architecture and distribution of heterogeneities: basement physiography, type and loci of carbonate production, bottom currents and changes in accommodation. Relative sea-level changes resulted from high-frequency glacioeustatic cycles that punctuated an overall tectonically induced transgression. Topographic relief, fragmentation of the limestone basement and wave energy controlled lithoclastic supply. Basement physiography and relative sea-level changes determined the area available for carbonate-producing biotas, and thus the amount and type of skeletal sediments. On the moderately dipping south-western margin, waves and associated currents re-distributed the lithoclasts derived from coastal abrasion. Bioclasts mostly derived from epiphytic seaweed production in the shoreface. Infralittoral prisms resulted from downdip transport (progradation) and longshore transport, which controlled along strike continuity of the prisms. Where basement continuity was interrupted at the margin of the palaeoisland, a lithoclastic fan was deposited from dumping sediments carried by shore-parallel currents. Fan-shaped skeletal bodies coalesced to form an apron on the steeper northern side of the palaeoisland, when a structural platform was flooded and epiphytic carbonates were shed. Red algae contribution depended on the available oligophotic area which, in turn, was controlled by high-frequency sea-level cycles, basement physiography and apron aggradation. The internal architecture of this apron resembles a low stand wedge but it accumulated during transgression. Deeper-water, contour-parallel currents formed a lithosome at the foot of the structural relief. Interpreted as drift deposits, it is composed of shallow-water benthonic skeletal components, planktonic foraminifera and some limeclasts
Foramol, rhodalgal and bryomol sediments are not necessarily temperate-water carbonates.
No full textEnvironmental factors influencing skeletal grain sediment associations: a critical review of Miocene examples from the western Mediterranean
No full textTropical foramol carbonates – Environmental factors influencing skeletal grain sediment associations
No full textSedimentological features of coarse-grained deep water backset beds in a distally steepened carbonate ramp (Late Miocene, Menorca, Balearic Islands, Spain)
No full textInternal Waves, an Under-Explored Source of Turbulence Events in the Sedimentary Record
No full textInternal waves occur nearly ubiquitously in lakes and oceans yet their sedimentary records remain largely unrecognized. Waves propagate at the interface between fluids of different densities. Surface waves propagate at the interface between air and water, which is a strong density gradient. Internal waves propagate along weaker gradients (pycnoclines) within density-stratified fluids, behaving similarly to surface waves but typically at lower frequencies and larger amplitudes. Internal waves that occur at tidal frequencies are called internal tides; they are very common on the outer continental shelf and slope, and are generated as the surface tides move stratified water up and down a sloping surface. Large internal solitary waves known as solitons are ubiquitous wherever strong currents and stratification occur in the vicinity of irregular topography. These waves can force short-period, strong bottom-current pulses and may trigger upslope-surging vortex cores of dense fluid (boluses) that can induce mobilization of bottom sediments.
Internal-wave deposits (internalites) are highly variable and definitive criteria for recognition are still to be developed. In terrigenous-clastic systems and shallow-water settings, internalites can be seen as “out-of-context” tempestites, detached from shore-related deposits and lacking thickening/coarsening upward sequences. In contrast to surface storm waves, the impact of internal waves is usually strongest in mid-outer-shelf regions and weaker in shallow water. Internal waves also provide a plausible mechanism to explain the origin of hummocky cross-stratification, especially their occurrence in different depositional environments. In deeper settings (continental slopes and canyons), internalites may have sedimentary structures indicating tidal currents and may coexist with turbidites. In carbonate systems, internal waves influence both sediment remobilization and the carbonate-producing biota. Differentiation between internal waves and surface storm waves is more reliable because many skeletal constituents have specific bathymetric distributions. Moreover, internal waves influence nutrient, plankton and larval distributions while inducing thermal variations by vertical displacements of the thermocline. The sharp gradient in nutrients and the chlorophyll-maximum zone typically correspond with the base of the seasonal pycnocline, which is commonly in the lower part of the photic zone. Suspension-feeding metazoans can thrive near the pycnocline, which explains the common occurrence of Phanerozoic metazoan buildups at mid- and outer ramp settings. During paleoceanographic changes that have influenced ocean stratification, internal waves may also have been a mechanism influencing diversification and extinction of these mid- and outer-ramp benthic communities
Reply to Shanmugam, G., Comment on Internal Waves, an Underexplored Source of Turbulence Events in the Sedimentary Record by Pomar Et Al. [Earth-Science Reviews, 111 (2012), 56-81], Earth Science Reviews (2012)
No full textShanmugam (2012) just published a criticism about the study of
Pomar et al. (2012) on the potential importance of internal waves
in shaping the sedimentary record in lakes and oceans. We would
like to acknowledge the effort done by G. Shanmugam for initiating
such a discussion. However, while scientific debates and controversies are necessary for the advancement of science in general, and for sedimentology in particular, they should be carried out with coherence and rigor to be constructive. Unfortunately, Shanmugam's comment appears to us not to follow this logic such that it is difficult for us to reply in a sensible manner
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