High-relief Carbonate Platform Research Articles

Facies character and depositional architecture of hydrothermal travertine slope aprons (Pleistocene, Acquasanta Terme, Central Italy)

Hydrothermal travertines develop various depositional geometries, from tabular to high-relief mounds or aprons with steep slopes, under the control of local topography, location and geometry of the vents, fault activity, hydrology, water physico-chemical properties, rates of thermal water flow and carbonate precipitation rates. This study focuses on two Pleistocene, tens of metres thick, travertine slope aprons accumulated on fluvial terraces in the Tronto River Valley (Acquasanta Terme, Central Italy) to investigate their facies character, geochemical signature, porosity and evolution of the depositional geometry through time.The two travertine aprons consist of four aggradational-progradational units, vertically and laterally stacked with onlap and downlap stratal terminations, separated by erosional unconformities produced by events of non-deposition and erosion, due to temporary interruptions of vent activity, shifts of the vent location and/or deviation of the flow directions. The travertine units include various depositional environments: 1) smooth slope, 2) terraced slope with metre-scale sub-horizontal pools separated by rounded rims and vertical walls, and 3) sub-horizontal, tens of metres wide ponds. Smooth slope clinoforms are made of centimetre to decimetre thick layers of crystalline dendrite cementstone, laminated boundstone and radial coated grain grainstone, precipitated by fast-flowing water on inclined substrates. Rims and walls of terraced slopes are built by crystalline dendrites and laminated boundstone. Sub-horizontal layers of terrace pools and ponds consist of facies precipitated by slow-flowing to standing water (clotted peloidal micrite dendrite, coated bubble boundstone, raft rudstone) associated with radial coated grains and laminated boundstone. Carbonate coated reeds occur in distal ponds adjacent to toe of slopes or overlie packstone/rudstone with travertine intraclasts and substrate extraclasts, marking events of subaerial exposure and erosion. Travertine facies porosity and permeability range from 4 to 21% and 0.03 to 669 mD, respectively, showing no direct correlation. Stable isotope values (δ13C: 5.7–9.3‰; δ18O −9.6‰ to −12.2‰) are similar to other travertines precipitated by thermal water in Central Italy.This study identifies the centimetre-scale travertine facies variability linking it to the environment of deposition and to the depositional geometry of travertine units affected by the substrate topography and lateral shifts of the active springs. Despite the different scale and facies composition, the geometry of aggradational-progradational units of travertine aprons might resemble marine flat-topped high-relief carbonate platforms. Travertine units in the subsurface, if present with sizes that can be seismically resolved, might be wrongly interpreted as carbonate platforms with steep slopes without a detailed facies analysis.

Sequence stratigraphy after the demise of a high-relief carbonate platform (Carnian of the Dolomites): Sea-level and climate disentangled

Sedimentary facies analysis aided by quantitative 3D georeferenced field data is applied to constrain the sequence stratigraphy of a complex stratigraphic interval in the Late Triassic of the Dolomites. This multidisciplinary approach was the key to disentangle the timing of climatic change vs. sea-level fluctuation and their effects on shallow water carbonate depositional systems. The “Carnian Pluvial Event”, a global episode of climate change worldwide documented at low latitudes, involved increased rainfall and possibly global warming. This climatic event begins before a drop of sea-level and caused the demise of microbial-dominated high-relief carbonate platforms that dominated the Dolomites region, and was followed by a period of coexistence of small microbial carbonate mounds and arenaceous skeletal–oolitic grainstones. A subsequent sea-level fall brought to the definitive disappearance of microbialites and shallow water carbonates switched to ramps dominated by oolitic–bioclastic grainstones. The crisis of early Carnian shallow water carbonate systems of the Dolomites generated a geological surface similar to a drowning unconformity, although no transgression occurred. As the high-relief microbial carbonate systems characterized by steep slopes switched to gently inclined oolitic–skeletal–siliciclastic ramps, basins were rapidly filled. The change of carbonate depositional systems was associated with an increase in siliciclastic input, in turn triggered by the onset of a humid climatic event and only later to a sea-level drop. This evolution of carbonate systems cannot be interpreted in the light of sea-level changes only: climate change, and consequent ecological changes in the main carbonate producing biotas, induced significant modifications in depositional geometries. This case study may serve as a conceptual model for the sedimentary evolution of carbonate systems subject to ecological crisis that do not evolve in platform drowning because, despite a drop in shallow water carbonate production, a combination of low subsidence and/or sea level drop maintains the platform top at shallow depth.

Demise of Late Triassic carbonate platforms triggered the onset of a tide-dominated depositional system in the Dolomites, Northern Italy

Facies analysis of the mixed carbonate–siliciclastic Upper Triassic Heiligkreuz Formation in the Cortina–Tofane area (Dolomites, Northern Italy) shows a tide-dominated sedimentary succession developed in a marine strait environment connecting two small sub-basins on the western margin of the Tethys Ocean. Both ebb and flood paleocurrents are preserved and their directions are in agreement with the strait orientation, as inferred from the geographic position of high-relief carbonate platforms in the substrate. Facies and sedimentary structures including planar cross stratification, herringbone cross stratification, flaser to wavy to lenticular bedding and cyclical alternations of mainly oolitic–siliciclastic vs. mainly bioclastic–muddy laminae related to variations of the hydraulic regime have been described and compared with other tide-dominated systems developed in confined marine conditions, and in particular with the Plio-Pleistocene deposits of the central Mediterranean Sea. However, in contrast with all other known cases, this tidal system did not develop in tectonically confined submarine grabens or canyons, but rather in a narrow marine passage between two relict high-relief carbonate platforms.

Syndepositional Deformation In A High-Relief Carbonate Platform and Its Effect On Early Fluid Flow As Revealed By Dolomite Patterns

Abstract Syndepositional fractures are common in Upper Permian strata of Dark Canyon, Guadalupe Mountains, New Mexico, USA, and are typically oriented parallel to the platform-margin trend with subvertical traces. Fracture apertures range from millimeter- to meter-scale with sediment, microbialite, breccia, and cement infill. Syndepositional normal faults are less frequent, and commonly are filled with fault-breccia and flanked by damage zones. Syndepositional fault offsets range from tens of centimeters up to 18 meters. In some instances, fault offsets were large enough to alter sedimentation patterns and stratal architecture of the Tansill shelf in the form of growth monoclines or small-scale grabens. Evidence for flow of dolomitizing fluids along syndepositional deformation features consists of narrow (centimeter- to meter-scale) dolomite halos along the fractures and faults, and dolomite bodies that extend laterally for several meters to tens of meters away from faults and fractures. Halos are interpreted to have formed where deformation features crosscut low-permeability strata. Dolomite bodies formed where deformation features intersected more permeable strata and dolomitizing fluids flowed laterally into the surrounding host rock. Isotopic data from individual fracture-controlled dolomite bodies indicate temporal variability in the dolomitizing fluids, suggesting that those fluids moved through different syndepositional fractures systems at different times.

Large-scale progradation, demise and rebirth of a high-relief carbonate platform (Triassic, Lombardy Southern Alps, Italy)

The Upper Anisian to Early Carnian succession of the Middle Val Brembana–Pegherolo Massif (Central Southern Alps of Italy) records a complete depositional cycle from platform inception to growth, demise and rebirth. The depositional architecture of this system reflects different evolutionary stages: an inception stage which postdates a previous drowning of an Anisian carbonate platform with progradation of the carbonate platform from the nucleation areas, an aggradational stage with increasing water depth in the basins, a progradational stage where steep slopes composed of margin-derived breccias develop and a final crisis corresponding to the subaerial exposure of the platform top, followed by the deposition of shales in the basin before the rebirth of a different type of carbonate factory. The record of this evolution reflects the effects of the change in accommodation space (interplay of subsidence and eustacy), which controls the type and storage sites of the sediments produced by the carbonate factory. The effects of the changes in accommodation space are recorded in the shallow water platform as well as in the intraplatform basins, where the sediments, delivered at different rates from the platform top are stored. As a consequence, the aggradational stage corresponds to reduced sedimentation in the basins (i.e. sediments are stored on the platform top) whereas during progradation resedimented limestones are more common in the basin. Subaerial exposure rapidly halted the carbonate production on the platform top, while a major input of shales (probably reflecting a climate change and/or lowering of the base level) is recorded in the basin, where shales onlap the slope of the previous carbonate system. The rebirth of the carbonate factory after subaerial exposure of the platform top is characterized by a different composition of the carbonate factory, probably reflecting changes of the environmental conditions. The step-by-step recording of the evolution of the carbonate system represents a unique opportunity to record a seismic-scale complete evolutionary cycle of a carbonate system in its different sub-environments, from the platform top to the basin.

Platform-Margin Trajectory as a Control on Syndepositional Fracture Patterns, Canning Basin, Western Australia

Abstract Syndepositional fractures are an important feature of high-relief, reef-rimmed carbonate systems and exert a profound control on many facets of platform evolution including: the timing and frequency of platform-margin collapse events, generation of an early diagenetic fluid flow system, and subsequent karst and enhanced-permeability development. In the Devonian Reef Complexes of the Canning Basin, syndepositional fracturing increases significantly with progradation, with a twofold increase in fracture intensity observed between the most retrogradational and progradational carbonate platforms. This study demonstrates a statistically significant relationship between syndepositional fracture patterns and variations in stratigraphic architecture, approximated here by platform-margin trajectory. In the platform-margin, syndepositional fracturing varies systematically with platform-margin trajectory, regardless of proximity to regional tectonic elements, whereas in the platform interior, tectonically active settings consistently display syndepositional fracture intensities significantly higher than predicted by platform-margin trajectory alone. The results presented here suggest a first-order relationship between long-term changes in stratigraphic architecture and syndepositional fracture development in high-relief carbonate platforms and imply that whereas external regional tectonic drivers may enhance early fracturing, they are not required.

FACIES DISTRIBUTION OF FUSULINIDA IN A BASHKIRIAN-MOSCOVIAN (PENNSYLVANIAN) CARBONATE PLATFORM TOP (CANTABRIAN MOUNTAINS, NW SPAIN)

The Sierra del Cuera (Cantabrian Mountains, Asturias, Spain) exhibits an exceptionally well-preserved upper Bashkirian-lower Moscovian succession of a high-relief carbonate platform with steep adjacent slope. During the late Bashkirian, the platform margin prograded rapidly basinward, and during the early Moscovian, platform aggradation increased. Fusulinida distributions have been estimated in three platform study windows representative of the Bashkirian and Moscovian lithofacies. Microbial boundstone accumulations at the platform margin and slope contain scarce fusulinoideans (particularly during the Moscovian aggradation, when water depth was relatively greater). Boundstones are characterized by tetrataxids, lasiodiscids, pseudoammodiscids, biseriamminids, endothyrids and palaeotextulariids. Grainstone units with sediments deposited above wave base are characterized by the predominance of Pseudoendothyra , Eostaffella (Bashkirian) or Profusulinella (Moscovian), Bradyina and palaeotextulariids. The open-marine facies, below effective wave-base but intermittently affected by storm currents, is enriched in endothyrids and palaeotextulariids in the Bashkirian part, and in Profusulinella , Eofusulina and Schubertella in the Moscovian part. The Moscovian facies, of lower energy and deeper water depth, contain some microbially precipitated micrite and have scarce Fusulinida, mostly represented by endothyrids, biseriamminids, and fusulinoideans ( Eostaffella , Ozawainella and Profusulinella ). Lagoonal open- to restricted-marine facies with beresellid algae contain Profusulinella , Schubertella and biseriamminids. Crinoidal packstones were deposited in moderate-energy settings close to the platform margin, below wave-base, and are characterized by Ozawainella , Eostaffella , palaeotextulariids and tetrataxids. These analyses show that fusulinid foraminifera inhabited mostly shallow and open-marine environments. Their distribution was controlled by depth-related variables, water energy and open-marine vs. restricted-marine conditions. Fusulinoideans increase in the Moscovian strata, and show different trends in the upper Bashkirian and lower Moscovian platforms, possibly because of: a) changes in the paleoecology of Fusulinida at the species level from the Bashkirian to the Moscovian; and/or b) variations in the depositional environments and physicochemical characteristics of seawater from the Bashkirian progradational phase to the Moscovian aggradational phase. Pseudoendothyra , Profusulinella , Schubertella , bradyinids and palaeotextulariids showed an affinity for high-energy settings. Profusulinella and Schubertella , however, preferred the shallowest facies and also tolerated restricted marine conditions. Eostaffella ’s distribution largely differs between the Bashkirian and Moscovian parts because it characterizes the high-energy grainstones of the upper Bashkirian and the open-marine, moderate- to low-energy environments of the lower Moscovian. Endothyrids were excluded from lagoonal environments with restricted circulation and abnormal salinity. Biseriamminids preferred open-marine, moderate- to low-energy settings with muddy substrates, but were not limited by water restriction. Greater tolerance to deeper, darker habitats was exhibited by lasiodiscids, pseudoammodiscids and tetrataxids, which occurred from the platform top down to 200 m depth on the Moscovian slope.

ABSTRACT: Prograding vs. Aggrading Slope to Platform Transitions in a Steep and High Relief Carbonate Platform Margin (Pennsylvanian, Northern Spain)

ABSTRACT: Prograding vs. Aggrading Slope to Platform Transitions in a Steep and High Relief Carbonate Platform Margin (Pennsylvanian, Northern Spain)

ABSTRACT: Prograding Steep and High Relief Carbonate Platform Margins

ABSTRACT: Prograding Steep and High Relief Carbonate Platform Margins

Mid‐Cretaceous basin margin carbonates, east‐central Mexico

ABSTRACTIsolated, high relief carbonate platforms developed in the intracratonic basin of east‐central Mexico during Albian‐Cenomanian time. Relief on the platforms was of the order of 1000 m and slopes were as steep as 20–43°. Basin‐margin debris aprons adjacent to the platforms comprise the Tamabra Formation. In the Sierra Madre Oriental, at the eastern margin of the Valles‐San Luis Potosi Platform, an exceptionally thick (1380m) progradational basin to platform sequence of the Tamabra Formation can be divided into six lithological units.Basinal carbonate deposition that preceded deposition of the Tamabra Formation was emphatically punctuated by an allochthonous reef block 1 km long by 0·5 km wide with a stratigraphic thickness of 95 m. It is encased in Tamabra Formation unit A, approximately 360 m of peloidal‐skeletal wackestone and lithoclastic‐skeletal packstone that includes some graded beds. Unit B is 73 m of massive dolomite with sparse skeletal fragments and intraclasts. Unit C, 114m thick, consists of structureless skeletal wackestone passing upward into graded skeletal packstone. Interlaminated lime mudstone and fine grained bioclastic packstone with prominent horizontal burrows are interspersed near the top. Unit D is 126 m of breccia with finely interbedded skeletal grainstone and burrowed or laminated mudstone. The breccias contain a spectrum of platform‐derived lithoclasts and basinal intraclasts, up to 10 m in size. The breccias are typically grain supported (rudstone) with a matrix of lightly to completely dolomitized mudstone or skeletal debris. Beds are up to several metres thick. Unit E is 206 m of massive, sucrosic dolomite that replaced breccias. Unit F is approximately 500 m of thick bedded to massive skeletal packstone with abundant rudists and a few mudstone intraclasts. Metre scale laminated lime mudstone beds are interspersed. The section is capped by El Abra Formation platform margin limestone, consisting of massive beds of caprinid packstone and grainstone with many whole valves.Depositional processes within this sequence shift from basinal pelagic or peri‐platform sedimentation to distal, platform‐derived, muddy turbidity currents with a large slump block (Unit A); through more proximal (coarser and cleaner) turbidity currents (Unit B?, C); to debris flows incorporating platform margin and slope debris (Units D, E). Finally, a talus of coarse, reef‐derived bioclasts (Unit F) accumulated as the platform margin prograded over the slope sequence. Interspersed basinal deposits evolved gradually from largely pelagic to include influxes of dilute turbidity currents.Units containing turbidites with platform‐derived bioclasts reflect flooding of the adjacent platform. Breccia blocks and lithoclasts were probably generated by erosion and collapse of the platform during lowstands. Laminated, black, pelagic carbonates, locally cherty, are interbedded with both breccias and turbidites. At least those interbedded with turbidites may have been deposited within an expanded mid‐water oxygen minimum zone during relative highstands of sea level. They are in part coeval with mid‐Cretaceous black shales of the Atlantic Ocean.