Evaporitic Drawdown Research Articles

Late Paleozoic seismic sequence stratigraphy and paleogeography of the paleo-Loppa High in the Norwegian Barents Sea

Abstract The paleo-Loppa High in the SW Barents Sea is a ridge structure, which developed during the late Paleozoic when the earliest phase of the Atlantic rifting between Greenland and Norway occurred. The southwest of the Barents Sea, located at the northern margin of Pangaea during the Carboniferous and Permian, was characterized by a structural style of half-graben geometries. The northward drift of the northern Pangaea triggered changes in regional climatic conditions that are reflected in the preserved sedimentary deposits. 2D/3D seismic combined with well and core data were used to define depositional seismic sequences and to understand the stratigraphic evolution of the paleo-Loppa High during the late Paleozoic. Based on the geometry of the defined seismic sequences and the character of observed sedimentary facies, a paleogeographic reconstruction of the key stages in the paleo-Loppa High evolution is also proposed and discussed in relation to local tectonic, global sea-level oscillations, and climatic changes. A total of seven seismic sequences, ranging from clastic-dominated to transitional clastic-carbonate sedimentation followed by an evaporitic drawdown phase, then shifting to carbonate-dominated sequences and finally capped by silica- and chert-dominated deposits, have been defined and represent the infill evolution of the paleo-Loppa High. Tectonics processes associated with the rifting are the principal controls in the 3-D morphology of the defined sequences. Sea-level fluctuations and climate changes have modified the biotic evolution and were responsible of the small-scale features inside each sequence. A renewed interest, in the study of the late Paleozoic sedimentary deposits of the paleo-Loppa High, has been manifested due to the recent discoveries of hydrocarbons in the Gohta and Alta prospects.

Stratigraphy of the Middle Devonian Keg River and Prairie Evaporite formations, northeast Alberta, Canada

Stratigraphy of the Middle Devonian Keg River and Prairie Evaporite formations, northeast Alberta, Canada

Topographic and sea level controls on oolite-microbialite-coralgal reef sequences: The terminal carbonate complex of southeast Spain

The terminal carbonate complex of southeast Spain is a Miocene (Messinian) unit of oolite, microbialite, and coralgal reefs deposited in association with glacioeustasy and evaporitic drawdown. The relationship between paleotopography and sea level history is useful for prediction of microbialite and oolite reservoir facies in the subsurface. Four sequences record sea level change with minimum amplitudes of 32–77 m (105–253 ft). Sequences commonly have local basal stromatolites overlain by local thrombolites, ooid grainstone, volcaniclastic-rich planar-bedded ooid grainstone, and fenestral ooid grainstone. At low substrate positions, thrombolite boundstones are thicker and laterally more continuous than at higher positions. At intermediate substrate positions (relative to sea level history), sequences have a build-and-fill architecture, characterized by a relief-building phase and a relief-filling phase, with thin sequences draping paleotopography. Microbialites dominate during the relative sea level rises and build topographic relief. Oolites dominate during the relative sea level falls and fill topographic relief. At higher substrate positions, close to highstand, sequences thicken and yield stratigraphic character that is inconsistent with a build-and-fill model. Apparently, the build-and-fill model requires an intermediate-elevation substrate position and nonoptimal carbonate productivity during rapid sea level change. Sequences progressively show increasing diversity and more normal marine organisms, possibly caused by decreasing aridity. Lithofacies of the La Molata area show evidence of more restricted conditions compared to the La Rellana-Ricardillo area lithofacies, likely because La Molata was in an embayment. These results show that distribution of oolite, microbialite, and reef facies are predictable given known interaction among sea level, paleotopography of the depositional surface (substrate), and coastline configuration.

The influence of paleogeography in epicontinental seas: A case study based on Middle Devonian strata from the MacKenzie Basin, Northwest Territories, Canada

The history of Middle Devonian sea level changes are recorded in the Chinchaga Formation, Lonely Bay Formation, Horn Plateau Formation, and the Horn River Formations that originated in the MacKenzie Basin, which is located in the southern Northwest Territories, Canada. These strata record an almost consistent sea-level rise throughout the Middle Devonian with punctuated deepenings. Deposition began in the MacKenzie Basin in the Eifelian with restricted shallow water evaporites and dolomites of the Chinchaga Formation. Overlying the Chinchaga Formation are the open-marine carbonates belonging to the Lonely Bay Formation, which were overlain by the Horn Plateau Formation buildups. Pelagic shales of the Horn River Formation in the Late Givetian/Early Frasnian later covered these buildups. During the Early and Middle Devonian many continental basins in North America had limited interactions with open ocean waters. These restricted shallow-water basins were vulnerable to the effects of clastic shedding and evaporitic drawdown that commonly caused local lowstands. Several of these shallow basins, including the MacKenzie Basin, have been used to construct Devonian sea-level curves. Comparison of the Middle Devonian strata that formed in the MacKenzie Basin with well-established sea level curves shows that the Late-Givetian regression interpreted from some of those successions is not evident in the succession found in the MacKenzie Basin. This probably reflects the fact that the Mackenzie Basin was directly linked to the open ocean and was not influenced by the changes in sea level that affected many of the enclosed basins.

Constraining controls on carbonate sequences with high-resolution chronostratigraphy: Upper Miocene, Cabo de Gata region, SE Spain

A high-resolution chronostratigraphy has been developed for Miocene shallow-water carbonate strata in the Cabo de Gata region of SE Spain for evaluation of local, regional and global factors that controlled platform architecture prior to and during the Messinian salinity crisis. Paleomagnetic data were collected from strata at three localities. Mean natural remanent magnetization (NRM) ranges between 1.53×10−8 and 5.2×10−3 Am2/kg. Incremental thermal and alternating field demagnetization isolated the characteristic remanent magnetization (ChRM). Rock magnetic studies show that the dominant magnetic mineral is magnetite, but mixtures of magnetite and hematite occur. A composite chronostratigraphy was derived from five stratigraphic sections. Regional stratigraphic data, biostratigraphic data, and an 40Ar/39Ar date of 8.5±0.1 Ma, for an interbedded volcanic flow, place the strata in geomagnetic polarity Chrons C4r to C3r. Sequence-stratigraphic and diagenetic evidence indicate a major unconformity at the base of depositional sequence (DS)3 that contains a prograding reef complex, suggesting that approximately 250 000 yr of record (Subchrons C3Br.2r to 3Br.1r) are missing near the Messinian–Tortonian boundary. Correlation to the GPTS shows that the studied strata represent five third- to fourth-order DSs. Basal units are temperate to subtropical ramps (DS1A, DS1B, DS2); these are overlain by subtropical to tropical reefal platforms (DS3), which are capped by subtropical to tropical cyclic carbonates (Terminal Carbonate Complex, TCC). Correlation of the Cabo de Gata record to the Melilla area of Morocco, and the Sorbas basin of Spain indicate that early–Late Tortonian ramp strata from these areas are partially time-equivalent. Similar strata are extensively developed in the Western Mediterranean and likely were influenced by a cool climate or influx of nutrients during an overall rise in global sea-level. After ramp deposition, a sequence boundary (SB3) in Cabo de Gata correlates with a sequence boundary in Morocco and a published third-order eustatic fall suggesting at least a partial eustatic control for the sequence boundary. Coral reefs began to develop earlier in Cabo de Gata than at Melilla or Sorbas, arguing for local factors affecting this major environmental transition. Later Messinian reefs (DS3) from all areas are time-equivalent, suggesting a regional or global control on their formation. Some Halimeda-rich horizons in the Western Mediterranean are not time-equivalent event strata as hypothesized by others. Correlation of the relative sea-level curve for the fringing reef complex (DS3) with a published eustatic curve suggests at least a partial third-order global eustatic control for the highstand part of the sequence. Downstepping DS3 reefs and initial subaerial exposure of earlier DS3 reef strata approximately correlate with initiation of a series of subaerial unconformities in the South Pacific. The longer-term relative fall in sea-level during DS3 downstepping reef progradation does not correlate with a published third-order eustatic fall. Eustatic sea-level fluctuations may have been associated with initiation of the Mediterranean Messinian salinity crisis, but the longer-term fall may have been linked to tectonic uplift in the Mediterranean region. Widespread distribution of ‘TCC-style’ cycles of approximately the same age suggests a regional (Western Mediterranean) or global control on sea-level change responsible for TCC cycles. In addition, four subaerial exposure-capped TCC cycles may correlate with similar subaerial unconformities in the South Pacific, suggesting at least a partial eustatic control on TCC cyclicity. The high rates of relative sea-level change needed to generate a minimum of 25–30 m sea-level changes associated with each cycle are consistent with glacio–eustacy along with rapid evaporitic drawdown in the Mediterranean.

Giant polygons in the Realmonte Mine (Agrigento, Sicily); evidence for the desiccation of a Messinian halite basin

ABSTRACT The Messinian Realmonte salt deposit consists of a 400-600 m-thick halite succession that can be divided into four main lith ologic units (base to top): A) cumulates of plates settled out from a stratified water column; B) plate cumulates in a shallowing-upward sequence containing six kainite layers near the top of the unit; C) cu mulates of skeletal hoppers with chevron overgrowths precipitated from a nonstratified water body; and D) skeletal halite and anhydrite. Spectacular vertical fissures cut through the upper part of unit B at its boundary with salt unit C. These fissures are spaced at intervals of up to 5 m apart, extend down to a depth of 6 m, and are filled by red mud. The salt beds affected by the fissures are commonly upturned (buckled) and are truncated and onlapped by the succeeding flat-lying halite beds of unit C. Additionally, the upturned layers are cut by vertical dissolution pipes that extend down to as much as a depth of 4 m below the fissured surface. To explain the observed superimposed structures we propose two mechanisms, both induced by the desiccation of the evaporite basin: (1) When the salt surface was exposed, the halite layers were buck led, broke into polygonal crusts, and formed tepee structures. The buckling was caused by a net volume increase due to thermal expan sion, and what is more important, precipitation of new halite induced by evaporative pumping of brines moving up from the groundwater table. The buckled salt layers were then affected by meteoric dissolu tion, which created vertical pipes. (2) The tepee structures and the associated zone of dissolution pipes are further crosscut by a new network of large polygons. These new polygonal sets are outlined by deep fissures in which red silt collected, blown by the wind or carried by episodes of surface flooding. This type of deep contraction crack is known to be produced by volumetric changes induced by annual temperature fluctuations. The documentation of contraction polygons and other exposure fea tures represents the first report of desiccation events during the de position of the Sicilian salt. No geological data are yet available to settle the questions if the salt of Sicily is correlatable with the halite present beneath the floor of the Mediterranean and if the desiccation of the Realmonte salt basin could have been induced by: a) local tectonic causes (uplift of the basin floor by thrust activity, b) simple evaporitic drawdown, c) basinwide drop of the Mediterranean sea level, or d) complex interaction among these factors.

Models of Evaporitic Drawdown for a Carbonate Source Rock: Middle Devonian, Saskatchewan

Models of Evaporitic Drawdown for a Carbonate Source Rock: Middle Devonian, Saskatchewan

Origin of Dolostone Reservoir Rocks, Smackover Formation (Oxfordian), Northeastern Gulf Coast, U.S.A. (1)

Formation of regionally extensive dolostone reservoir rocks in the Smackover can be understood despite the possible effects of recrystallization. Geochemical and petrographic data suggest that dolomitization took place in (1) seawater-seepage, (2) reflux, (3) near-surface mixed-water, (4) shallow-burial mixed-water, and (5) deeper burial environments, which overlapped in time and space to form a platform-scale dolostone body composed of a complex mixture of dolomites. Seawater-seepage and reflux dolomitization occurred in the near surface penecontemporaneously with deposition of the Smackover and overlying Haynesville Formations. Dolomitization by seawater seepage occurred within an oolite grainstone sill which separated an intraplatform salt basin from the open sea. Seawater flowed landward through the sill in response to evaporitic drawdown of brines in the isolated intraplatform basin. Isolation of the salt basin occurred during the Oxfordian when the shoreline retreated from the Conecuh embayment. Dolomite located at the top of the Smackover enriched in {18}O suggests additional dolomitization by reflux of hypersaline brines. Reflux occurred as Buckner coastal sabkhas prograded over Smackover oolite grainstone shoreface deposits. Vugs lined with shallow-burial calcite and dolomite cements indicate flushing of the Smackover grainstone aquifer with fresh water. Freshwater intrusion probably occurred following sea level lowstands during the Late Jurassic and Early Cretaceous. Leaching in the proximal portion of the freshwater aquifer produced excellent limestone reservoir rocks in the updip Smackover. Dolomitization in the contemporaneous downdip mixed connate/freshwater zone formed dolostone reservoir rocks with depleted isotopic compositions consistent with a shallow-burial mixed-water origin.

Morphology from subaerial erosion of a Mediterranean seamount

The morphology of a seamount in the Valencia Trough of the Western Mediterranean has been investigated with side-looking sonar (Sea MARC I) and swath-mapping sonar (Sea Beam). The seamount is characterized by deep reentrants on its flanks, radiating spurs and a broad summit plateau. Presently its summit lies 1020 m below sea level. Avalanche scars and debris flow deposits are detected on re-entrant floors and slopes. Fragments of aphanitic basalt and andesitic tuff, sampled at DSDP Site 122 attest to the volcanic origin of the Valencia Seamount. First constructed as a subsea volcano about 20–25 m.y. ago, its flanks became almost entirely emerged 6 m.y. ago at the time of the evaporitic drawdown of the Mediterranean. Valencia Seamount has some features in common with young and old seamounts on the floor of the Pacific Ocean that have never emerged above sea level and other features which may be attributed to subaerial exposure. Two working hypotheses, erosional and constructional, are presented to explain the observed morphology. The erosional hypothesis better accounts for the known paleogeographic and paleoceanographic history of the Mediterranean. The mapped spurs are interpreted as flank rift zones which give many seamounts a starfish-like appearance. Fountain eruptions of hyaloclastites from vents could explain the broadly arched summit plateau. Peaks might be secondary parasitic cones or remanent erosional features of stocks and dykes which are more resistent to mass-wasting than an extrusive cover of vesicular lava flows and deposits of tuff and ash. Gullies are most abundant on the middle and lower flanks. Whereas the gullies might be breached calderas, it is more reasonable to expect that they are dominantly subaerial erosional in origin, considering the emergence history of this particular seamount.

Messinian subaerial erosion of the proto-Nile Delta

The evaporitic drawdown and subsequent dessication of the Mediterranean basins during the Messinian salinity crisis has been confirmed by detailed seismic mapping of the ensuing erosion surface in the Nile Delta region. The basal Messinian unconformity, determined by seismic analysis and borehole correlation, extends from almost sea level in the southern delta area to depths of at least 5 km in the Nile Cone. The onlap of the contemporaneously deposited fluvio-deltaic clastics of the Qawasim Formation occurs at between 2.5 to 4 km below present. Gradual initial drawdown of the Mediterranean Sea with occasional sudden sporadic falls, is implied by the presence of entrenched meanders and impaired rejuvenation terraces in the Nile Canyon, formed as the proto-Nile River cut down into the pre-existing Tortonian delta complex. The former Tortonian delta slope became an extensive linear erosional front with over 3 km of vertical relief, the uppermost levels of which were protected by tabular Oligocene basalt flows, preserved as mesas and buttes. The lower slopes were the site of coalescing piedmont fans fed by upper slope wadi systems. The piedmonts merged into the Qawasim fluvio-deltaic deposits of the proto-Nile fluvial flood plain. The subcrop beneath the basal Messinian unconformity reveals a remarkable diversity of rock type, each with its own distinctive control on the geomorphology and geometry of the ensuing Messinian erosional surface. Fracture patterns within the Oligocene basalts produced a trellis drainage network which became later captured by a dendritic system developed on less resistant prodelta shales of the former Tortonian delta slope. Close study of seismic profiles clearly demonstrates that the basal Messinian erosional disconformity can be traced northwards to depths of at least 2.5–3.0 km before becoming affected by later Pliocene faulting. This validates a deep dessication model for the evolution of the Messinian evaporites.

Mesozoic and Cenozoic Rocks from Malta Escarpment (Central Mediterranean)

Sedimentary rocks of Triassic-Neogene age are present on the Malta Escarpment of the eastern Mediterranean. Upper Triassic dolomitic limestones of shallow-water origin, at depths between 2.5 and 3.5 km, are similar in lithofacies to coeval platform carbonates of the Siracusa (Syracuse) belt of southern Sicily. Jurassic rocks include lower-middle Liassic shallow-water limestones followed by condensed hemipelagic lime deposits indicative of sinking and starving of the former platform. Cretaceous materials are represented by both red marls rich in planktonic faunas and reworked volcaniclastic breccias including shallow-water skeletal material. Paleogene rocks are both shallow-water limestones with corals, algae, and bivalves, and redeposited calcarenites of lithofacies similar to those from surface and subsurface of the Ragusa zone. Oligocene-?lower Miocene rocks from the escarpment are also similar in lithology to the coeval Ragusa deposits. Tortonian is represented by hemipelagic marls indicating open-marine environment. Pervasive dolomitization on lime crusts and on initial-stage fissure fillings with strongly positive isotopic oxygen ratio is thought to be a product of Messinian evaporitic drawdown. Pliocene sediments belong to the Trubi facies and consist of pelagic foraminiferal chalk. An impressive vertical relief existed by Miocene times, as attested by Messinian crusts and veins on or in rocks as old as Late Triassic. Our data do not provide evidence that this morphologic feature necessarily coincides with a continent-ocean transition. The present escarpment was produced by faulting, erosion, and defacement.

The nature and distribution of Messinian erosional surfaces — Indicators of a several-kilometer-deep Mediterranean in the Miocene

The deep-basin desiccation model for the origin of Messinian-age Mediterranean evaporites implies two basic assumptions: 1. (1) The evaporites were in part laid down under shallow-water conditions, with intermittent subaerial episodes brought about by evaporitic drawdown. 2. (2) Some of the depressions in which the evaporites were deposited were already several kilometers deep during Messinian time, and hence, the general configuration of these basins predated, and was essentially independent of the salinity crisis. The first assumption has been more or less accepted by the scientific community as a consequence of the discovery of supratidal to intertidal features in the evaporites, whereas there has been less acceptance of the second assumption. The arguments on which the second assumption was first formulated, were multiple and included: (a) the presence in seismic reflection profiles of large pre-salinity-crisis depocenters with sediment-distribution patterns not substantially different from those of the post-salinity-crisis strata; (b) systematic changes in the evaporitic facies along basin-slope transects indicative of existing paleorelief; and (c) the occurrence of continental-rise and abyssal-plain-type deposits in both pre- and post-salinity-crisis successions. New attention is herein focused on Messinian-age erosional surfaces created by evaporitic drawdown of an isolated Mediterranean Sea. These erosional surfaces are detected as discordances in seismic reflection profiles. The discordances can be traced from the subsurface of the present coastal plains and continental shelves to the subsurface of the modern abyssal plains and show entrenchment into the underlying strata. On the passive-type continental margins, like those of the Balearic Sea and those in the southeastern corner of the Levantine Sea, the seaward gradients of the erosion surfaces, corrected for post-Messinian isostatic sediment loading and compaction and adjusted for regional subsidence, permit the calculation of a former relief between the pre-salinity-crisis shore-line and basin center of more than 2.5 km for the western Mediterranean and more than 3.0 km for the eastern Mediterranean.

Southern Alpine lakes — Hypothesis of an erosional origin related to the Messinian entrenchment

All the major lakes lying south of the Alps (Maggiore, Lugano, Como, Iseo and Garda) are cryptodepressions, unlike the northern Alpine lakes. Their glacial origin is discussed and refuted on the basis of geological, structural and geomorphic arguments. A major erosional phase is recorded south of the Alps after the deposition of the Gonfolite Formation (a synorogenic to postorogenic sedimentary wedge representing a deep-sea fan, ranging in age from Late Oligocene to Middle Miocene) and prior to the Pliocene transgression. The deposits of the Gonfolite have been deeply incised near Como: a tentative reconstruction of the inferred Messinian paleocanyon is proposed. The erosional surface can be followed under the Po Plain by means of seismic reflection profiles and is controlled by hundreds of deep exploratory wells. According to the available surface and subsurface data, the erosional phase can be dated at the Late Miocene (Messinian): it is related to the dramatic drop in sea level resulting from the isolation of the Mediterranean basin(s) from the world ocean at the close of the Miocene. The depressions presently occupied by the southern Alpine lakes are interpreted as canyons deeply incised along either former paleovalleys (as is the case for Lake Como) or along tectonically controlled embayments (as is the case for Lake Garda) during the evaporitic drawdown. The depressions created by the entrenchment have been eventually occupied and partly reexcavated by glaciers during the ice ages. Frontal moraines dammed the southernmost edge of the valleys, which are presently occupied by the lakes.

Carbonate-Anhydrite Facies Relations in Otto Fiord Formation (Mississippian-Pennsylvanian), Canadian Arctic Archipelago

The Otto Fiord Formation is a major evaporite deposit of Late Mississippian to Middle Pennsylvanian age occupying an axial position in the Sverdrup basin of the Canadian Arctic Archipelago. Where exposed in normal stratigraphic succession on northwestern Ellesmere Island, the Otto Fiord is composed of 350 to 600 m of rhythmically alternating limestone and anhydrite, with some interbedded sandstones near the top of the formation. The formation is underlain by continental to marginal-marine red beds, and is overlain by carbonate rocks and shales of deep-water aspect. Laterally, the Otto Fiord evaporites and limestones are coeval with marine-shelf carbonate rocks that flank the margins of the evaporite-carbonate basin. In the central and southern Sverdrup basin, a salt and anhydrite facies of the Otto Fiord is buried by thick Mesozoic rocks and is revealed at the surface only in large diapiric structures. At several localities on northwestern Ellesmere Island, limestone mounds are enclosed within the Otto Fiord Formation. At van Hauen Pass, mounds up to 30 m thick and 350 m long are built on erosional remnants of anhydrite. The mounds have a crinoid-rich marine limestone base, a main beresellid algal-buildup facies of hypersaline aspect, and several marine-limestone capping beds. Each of the marine-limestone phases has thin, marine-limestone equivalents in the intermound setting, each separated by a unit of anhydrite. Reconstructions of depositional events reveal that the mounds are the composite record of at least three transgressive-regressive cycles in which carbonate sediment was deposited during the marine phase and the algal facies and anhydrite units were formed under the hypersaline conditions imposed by evaporitic drawdown. At a facies transition from Otto Fiord evaporites to the basin-margin reef and shelf carbonate rocks exposed on northwestern Ellesmere Island, anhydrite units are interbedded with limestones at the foot of steeply dipping shelf-foreslope carbonate tongues. Depositional relief on these carbonate units increases upward through the evaporite succession from a few meters to more than 400 m. Anhydrite units interbedded with the limestones apparently formed subaqueously under conditions of depressed hypersaline sea level, probably shallower than the depths suggested by carbonate depositional relief, but far from a sabkhalike environment. The accumulated evidence supports earlier interpretations that the anhydrite evaporites of the Otto Fiord Formation were deposited in a hypersaline subaqueous environment under conditions of regional evaporative drawdown of sea level. The complex facies relations demonstrated by the Otto Fiord rocks may serve as guides for modeling evaporite-reef relations and major carbonate to evaporite-facies transitions in other basins and in subsurface studies related to hydrocarbon exploration.