Shallow-water Evaporites Research Articles

2‐D Geodynamic Modeling of the Central South Atlantic Wide Rifted Margins, Implications for Evaporite Deposition

AbstractThe thick late syn‐ to early post‐rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn‐rift evaporites in the central South Atlantic are not well constrained and remain to be quantified. We use forward 2‐D thermo‐mechanical modeling coupled with melt prediction and surface processes to assess the contribution of lithospheric and mantle processes on the distal margin topography and subsidence history during continental rifting. Models show that (a) counter‐flow of depleted lower lithospheric mantle during rifting explains the magma‐poor nature of these margins and (b) weak crust and syn‐rift sediment control the wide crustal necking and subsidence history of the distal margin. Integration of our modeling results with quantified geophysical and geological observations suggests that (a) base level was down to −600 m below present‐day global sea level (bsl) during distal margin formation in the Aptian before sag and evaporite deposition, (b) base level was about −300/−400 m bsl at the end of evaporite deposition, and (c) scenarios with a fixed shallow base level (−400 m bsl) or with an increasing base level from an initially deep position (−1,600 m bsl) during evaporite deposition can both fit the observed evaporite distribution. However, erosional features along the base of evaporites suggest a deep initial base level.

Carbonate/evaporitic sedimentation during the Messinian salinity crisis in active accretionary wedge basins of the northern Calabria, southern Italy

Abstract This work deals with Messinian deposits belonging to the Neogene infill of the Rossano and Belvedere Basins, respectively developed along the fore-arc and the back-arc areas of the north Calabria accretionary wedge. The main goal is to characterize the carbonate and evaporitic sedimentation during the Messinian Salinity Crisis, in the general framework of the basin architecture and the interplay between eustatic vs tectonic controlled sea-level variations. Fieldwork integrated with seismic lines and well logs interpretations led to the revision of the general stratigraphy of the basins and the proposal of a new sequential stratigraphic model driven by cyclic sea-level variations. Each cycle, repeated at least two times during the Messinian Salinity Crisis time frame, begins with a relative sea-level fall responsible for the emplacement of prograding wedges composed of terrigenous and evaporitic deposits that, subsequently, evolve in the deposition of primary basin-fill evaporites. This phase is followed by open marine transgression due to relative sea-level rise that terminates the evaporite formation and predates the development of microbial dominated carbonate platforms associated with shallow-water evaporites. Both basins experienced intense tectonic activity during the Messinian, which could be responsible for huge basinward sediments exportation and fast decreasing in the accommodation space. However, this did not substantially influence the development of the systems tracts that, considering the basinal architecture, have been mainly controlled by eustatic sea-level variations. In fact, two major sea-level drops associated with basin restriction and aridity (cold) conditions seem to have caused the origin of two main evaporitic units as basin-fill evaporites, while consequent sea-level rises and less stressed condition, account for two carbonate units with limited evaporites and terrigenous deposition.

A deep palaeovalley in the floor of Polish Carpathian Foredeep Basin near Pilzno and its control on Badenian (Middle Miocene) evaporite facies

The Pogorska Wola palaeovalley of combined tectonic and erosional origin dissects the Mesozoic floor of the Carpathian Foredeep Basin to a depth exceeding 1200 m. It formed during Paleogene times presumably due to fluvial and submarine erosion, concentrated along a local pre-Late Badenian graben system. All members of the foredeep’s Badenian-Sarmatian sedimentary fill attain distinctly greater values inside the palaeovalley than on top of elevated plateaux on palaeovalley shoulders. The fill comprises the Early to Late Badenian sub-evaporite Skawina Formation, the laterally equivalent Late Badenian evaporite Krzyzanowice and Wieliczka formations and the supra-evaporite Late Badenian to Early Sarmatian Machow Formation. Over the plateaux and in the highest palaeovalley segment, the evaporites are developed in the sulphate facies Krzyzanowice Formation, whereas in the lower palaeovalley segments chloride-sulphate facies evaporites of the Wieliczka Formation occur. The rock salt-bearing rocks are involved in thrusting and folding at the Carpathian orogenic front, which helps to assess the lateral extent of the Wieliczka Formation in seismic records. The deep palaeotopographic position of the evaporites inside the palaeovalley, combined with their lithological and sedimentary features, point to their formation via subaqueous gravity flow-driven redeposition of originally shallow-water evaporites, preferentially halite-bearing, presumably combined with precipitation from sulphate and chloride brines at the palaeovalley floor. Both the redeposited sediments and the brines must have come from the adjacent plateaux and from a thrust-sheet top basin, approaching from the south on top of the Cretaceous-Paleogene Carpathian flysch thrust wedge

Depth indicators in Permian Basin evaporites

Abstract The Permian Basin of West Texas and New Mexico contains one of the world's best-preserved and most extensively studied evaporite basin-to-platform sequences. From analysis of fabrics and small-scale cycle patterns, reconstruction of the position of these elements in the basin-filling sequence and comparison to laboratory-grown and modern evaporite fabrics, we created a table of fabrics that serve as water-depth indicators. Evaporites formed in deeper water (more than a few to hundreds of metres) in both halite- and gypsum-precipitating settings in the Permian Basin are characterized by cumulate fabrics. Cumulates are fine crystals or rafts of fine crystals formed at the air – brine interface that fall though the water body and accumulate on the basin floor with fine lamination, draping relationships, dark colours and minimal early diagenesis. Intervals of coarser crystals precipitated on the basin floor are interpreted as evidence for episodic transport of saturated surface water to the basin floor during perturbation of stratified conditions. Shallow water evaporites in the Permian Basin are dominated by bottom-growth fabrics such as halite chevrons and near-vertically oriented gypsum crystals. Bands of fluid and other inclusions record high frequency changes in depositional rate. Truncated crystals document flooding by undersaturated fresh or marine water under shallow conditions where mixing was adequate to cause undersaturated low-density water to contact the basin floor. Formation of base-of-cycle insoluble residues is a strong indicator of shallow water during the flooding event that initiated each sedimentary cycle. In the Permian Basin, exposure is documented by formation of synsedimentary evaporite karst pits and pipes, truncation, dissolution and recrystallization of earlier fabrics, and precipitation of cements. Red siliciclastic mudstones are associated with the late stages of depositional cycles when the surface was subaerially exposed. Repetition of alternately exposed and saline water table conditions created an array of distinctive fabrics including chaotic mudstone–salt mixtures, karst fills, replacement and recrystallization fabrics, and cracks and saltpolygons.

Offshore evidence of polyphase erosion in the Valencia Basin (Northwestern Mediterranean): Scenario for the Messinian Salinity Crisis

The Messinian Salinity Crisis at the end of the Miocene strongly affected the physiography of the Mediterranean margins, and in particular that of the Valencia Basin and its margins. The Valencia Basin, which is an aborted Tertiary rift related to the opening of the oceanic Provençal Basin, shows evidence of two major Messinian erosion surfaces — a Basal Erosion Surface (BES) and a Top Erosion Surface (TES) — bracketing the Messinian evaporite succession; the basin's margins show only a single erosion surface, here termed the Margins Erosion Surface (MES). In examining the geometrical relationships of these erosion surfaces with the deep basin facies deposited in the abyssal Provençal Basin during the Messinian Salinity Crisis, we have been able to establish a chronology for the Messinian in the Valencia Basin. Briefly, the Basal Erosion Surface extends the Margins Erosion Surface into the Central Valencia Basin and records an early and important sea-level fall at the beginning of the Messinian Salinity Crisis. The Miocene basin series were incised by canyons, while the exposed margins underwent substantial subaerial erosion. The basin's detrital products linked to this basal erosion grade laterally into the massive Messinian Salt layer of the Provençal Basin. The overlying Upper Evaporites unit appears to be widely transgressive beyond the edge of the Messinian Salt and onlaps the Basal Erosion Surface in the Valencia Basin. This disposition suggests a progressive sea-level rise during the deposition of the Upper Evaporites, possibly due to the rapid accumulation of thousands of metres of shallow-water evaporite facies in the Provençal Basin. The Top Erosion Surface at the top of the Upper Evaporites shows a network of incisions, and we suggest that this final erosion was linked to the Lago-Mare event recorded elsewhere in the Mediterranean at the end of the Messinian Salinity Crisis.

Carbonate deposition and hydrocarbon reservoir development at the Precambrian–Cambrian boundary: The Ara Group in South Oman

The Ediacaran–Early Cambrian Ara Group in the South Oman Salt Basin consists of six evaporite–carbonate cycles (A0/A1 to A6) that record tectono-eustatic sea-level changes. The A4 cycle developed at the Precambrian–Cambrian boundary (∼ 542 Ma). It forms a single depositional sequence with evaporites deposited in a lowstand systems tract (LST). Overlying carbonates represent an open-marine shallowing-upward ramp succession that developed in transgressive (TST) and highstand systems tracts (HST). The low-energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high. The TST facies include sulfates, evaporite–carbonate laminites, and organic-rich carbonate laminites that record initial flooding, deepening of the basin, and establishment of an outer ramp depositional environment. Carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity. Subsequent HST facies comprise mostly fine-grained clastic carbonates and stromatolites that formed in middle and inner ramp settings. These facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp. Sediment was redistributed in suspension, by muddy turbidity currents, muddy debris flows, storm and shallow-water currents. During the late HST, carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying LST evaporite unit. A combination of strong tectonic subsidence and transient flooding caused significant shallow-water evaporite deposition to occur not only down dip, but also on top of the former carbonate platform, where several hundreds of meters of evaporites accumulated. The transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit. The presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties. Distribution of organic material and carbonate mud can be linked to specific environmental conditions (low physical and biogenic disturbance of sediment and stratified water mass) and the sequence stratigraphic position (low flux of fine-grained carbonate during TST). In contrast, diagenetic evaporite formation has largely degraded reservoir quality in porous shallow-water facies near the top of the A4C.

The Mediterranean Messinian salinity crisis: an Apennine foredeep perspective

Owing to its expanded stratigraphic sections, the Apennine thrust belt offers the opportunity to better understand the evaporitic and post-evaporitic Messinian events. A physical stratigraphic framework of Messinian deposits, based on facies analysis and basin-wide correlation of key surfaces and sedimentary cycles, is presented. It is shown that the Messinian Apennine foredeep had marginal basins with shallow-water primary evaporites and deeper basins where resedimented evaporites accumulated under relatively deep-water conditions. Like many other Mediterranean examples, primary shallow-water evaporites of Apenninic marginal basins show evidence for subaerial exposure and erosion. However, the development of such an erosional surface does not correspond to the deposition of primary evaporites in the deepest part of the basin(s); here, the unconformity can be traced towards the base of resedimented evaporites or to a level within them, implying that the deeper basins of the Apennine foredeep never underwent desiccation during the Messinian salinity crisis, but rather received the eroded marginal evaporites. This fact, usually overlooked, raises important questions about the deep desiccation model of the Mediterranean.

Source rock potential of shallow-water evaporites: An investigation in holocenepleistocene Salt Flat sabkah (playa), west Texas-New Mexico

Source rock potential of the shallow-water evaporite-carbonate sequences at Salt Flat sabkha (playa) was evaluated on the basis of total organic carbon TOC), extractable organic matter and Rock Eval pyrolysis. Sedimentological evidence indicates periodic anoxicity in the Salt Flat brine lake during evaporite deposition. This was caused during wetter intervals when inflowing runoff rested on denser brine, forming a halocline. Brine stratification inhibited free circulation of oxygen in the lake bottom, enhancing the preservation of organic matter. Organic matter in gypsum-rich horizons is dominantly of algal origin. Average organic carbon content of the sediments is 0.6% but may be 1.2% or more in gypsum-rich sediments. Hydrogen Index is generally high, ranging from a low of 300 mg HC g/Corg for gypsum-rich beds.

Shallow-Water Evaporite Cycles in the Middle Devonian of Western Canada

Shallow-Water Evaporite Cycles in the Middle Devonian of Western Canada

Desert gypsum crusts as palaeoenvironmental indicators: A micropetrographic study of crusts from southern Tunisia and the central Namib Desert

Desert gypsum crusts are of three main types: shallow-water evaporites, which are characterized by size-graded beds; subsurface crusts, which are either macrocrystalline groundwater evaporites, or mesocrystalline illuvial accretions; and surface crusts—excluding the bedded type—which are exhumed illuvial crusts. Pedogenic processes involving the leaching of gypsum-rich surface deposits and subsequent displacive gypsum crystallization in the lower soil zone account for the formation of the illuvial crusts. The various forms of surface gypsum crust—columnar, powdery, and cobble—represent different stages in the degradation of exhumed crusts. Dissolution and leaching of gypsum transform columnar crusts to cobbles which deteriorate leaving a powdery residuum. Examination of thin sections of gypsum crusts from southern Tunisia and the central Namib Desert reveals that the exposure and degradation of illuvial, pedogenic crusts are characterized by distinct diagenetic features. The subsurface, illuvial crusts are composed predominantly of mesocrystalline (50 μm to 1·0 mm in diameter), lenticular gypsum. Crystals larger than 1·0 mm sometimes have poikilitic inclusions, though these features are more common in the macrocrystalline groundwater crusts. Rarely, fibrous gypsum crystals—indicative of displacive crystallization—are found in the illuvial crusts. Syndepositional and early diagenetic features include biogenic structures and calcite pseudomorphs after lenticular gypsum. Columnar surface crusts are composed predominantly of albastrine gypsum (crystallites less than 50 p.m in diameter). This texture develops through rapid gypsum crystallization following partial dissolution of lenticular crystals by infiltrating meteoric water. When large crystals occur as isolated remnants (porphyroblasts) in the surface crusts, they typically exhibit dissolution features and, occasionally, alabastrine overgrowths. The recognition of these characteristic structural and textural features in relict gypsum crusts can provide detailed information about the geomorphic history and palaeoenvironments of many arid regions. Radiometric dating of gypsum palaeosols is problematic because of their illuvial origin. Nevertheless, evidence suggests that the gypsum crusts of southern Tunisia developed in the early Holocene following desiccation of the chotts which had been inundated during the late Pleistocene. Evaporation of the lake waters precipitated gypsum which was then deflated and redeposited on the surrounding landscape. From here it was leached into the lower soil zone and illuvial gypsum crusts accreted. Subsequently, these pedogenic crusts were exhumed and the gypsum is currently being leached into the lower soil zone once again. In the central Namib Desert, the ages of the gypsum crusts are uncertain. Notwithstanding the difficulty in obtaining radiometric dates, studies of the gypsum's isotopic composition have provided valuable information on palaeoenvironments, and are a promising area of future research. Some of these studies have suggested that the crusts in the Namib are recent features. However, several 14C dates and also calculations of the theoretical rates of accretion indicate that the crusts may have formed during the late Pleistocene. If this is the case, their preservation supports the view that the Namib Desert has remained arid throughout the late Pleistocene and Holocene.

Shallow-water evaporitic environments and their source rock potential

In the major evaporitic environments on the world's surface today, most organic matter accumulates in shallow, subaqueous to seasonally, subaerially exposed algal-mat sediments. Given the present depositional setting and the most likely flushing by oxidizing water, it is unlikely this organic matter could be preserved to form source rocks. However, if we place such evaporite deposition into a geologic context, then there are times in the past when source rocks could have formed in shallow-water settings. Such settings were characterized by hydrological conditions which allowed the retention of hypersaline, anoxic pore waters to depths where the organic material was buried deeply enough to generate hydrocarbons. When deep-basin, shallow-water evaporite successions were laid down in basins such as the Mediterranean in the Late Miocene, the Michigan Basin in the Silurian, and other large saline giants, then conditions were right for the formation of source rocks within shallow-water and salt-flat evaporitic environments. The evaporites in these saline giants were deposited under conditions of relatively shallow water (< 50 m); the basin never appears to have dried out, but water levels changed quickly ( nearly equal 10,000 years) from shallow to deep. Continual water saturation coupled with saline pore fluids prevented the inflow of fresh, oxidizing groundwater into the basin center of shallow-water, organic-rich evaporites. Immature hydrocarbons derived from such rocks today drip from the 5.5 m.y. evaporites of Sicily in active salt and sulfur mines. Organic-rich sediments could also be preserved to generate hydrocarbons in rapidly subsiding rift basins. In such basins, rapid burial has prevented the entrance of fresher oxygenated waters and the associated degradation and destruction of the organic matter. The early continental riff stage generates the source rocks; the ephemeral streams, wadis, and dune fields generate the reservoirs; and the subsequent evaporite stage seals the reservoir.

Messinian badlands on the southeastern margin of the Mediterranean Sea

Substantial subaerial denudation of the North African continental margin took place during the Late Miocene salinity crisis. The Nile drainage was deeply entrenched as far as 1200 km inland, upto the Aswan cataract. Beneath the modern birdfoot delta the thalwegs of ancient Messinian river valleys lie as much as 2.5 km below present sea level. The relief between the top of the incised Late Miocene coastal plain and the axis of excavated channels exceeds 1.2 km. In the subsurface of the lower reaches of the modern Nile Cone a network of bifurcating entrenched channels descends to 4.6 km below sea level to become covered by the southern limit of the Messinian Evaporite Formation. A paleodrainage system is also present beneath the Levantine margin. Seismic reflection profiles document an extremely rough and steeply gullied morphology which is analogous to the badlands of South Dakota. In addition, reflection profiles reveal uniform and gentle seaward gradients of the preerosion (Jurassic to Late Miocene) and posterosion (Pliocene and Quaternary) sedimentary successions beneath the coastal plain and continental shelf. The closely comformable nature of all these depositional surfaces strongly indicates that the badland topography does not owe its origin to an epiorogenic uplift and tilting of the continental edge, but, instead, to a lowering of base level brought about by an unprecedented eustatic drop in water level within isolated Mediterranean basins. Calculations which consider the progressive isostatic loading of both the onshore and offshore sedimentary prism indicate a fall of the base level of at least 3.5 km. This accounts for the observed depth in the Levantine Basin at which Messinian-age shallow-water evaporites lap against the contemporaneous North African subaerial erosional surface.