Geology of the Sahil 1:100 000 map sheet, 100-17, United Arab Emirates
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Abstract:
This Sheet Description describes the Quaternary and solid geology of the Sahil 1:100 000 scale geological map of the UAE. The Sahil district covers 2500 km2 of the country stretching inland from the coastal sabkha. The land rises from 10 m above sea level on the coastal plain in the northwest to over 100 m in the southeast. Miocene bedrock crops out beneath the northern half of the district, forming isolated jebals up to about 30 m high within fields of low barchanoid dunes, and underlying intervening interdunes or small sabkhas.
The oldest rocks in the area are the Miocene rocks of the Dam, Shuwaihat and Baynunah formations. The Dam Formation comprises pale grey, fine grained carbonates and nodular gypsum beds which crop at low elevations within and around the margins of the coastal sabkha. The overlying Shuwaihat Formation is characterised by red sandstones with subordinate green mudstones and grey pedogenic beds laid down in a mixed aeolian-fluvial-sabkha environment. It is overlain by the Baynunah Formation which has a characteristic basal bone bed with an erosive base. Above is a sequence of interbedded calcareous siltstones, siltstones and fine sandstones, laid down in a low-energy fluvio-lacustrine system (Barakah and Hamrah members), capped by reddish brown, playa or lacustrine sandstones (Sahil Member).
Miocene deposits are overlain unconformably by Quaternary deposits of the Madinat Zayed Formation: quartz dominated, cross bedded aeolianite and lacustrine siltstone, and the Ghayathi Formation: moderately cemented, carbonate dominated aeolianite.
Much of the district has a cover of unconsolidated aeolian sand either as a thin veneer within the interdunes and coastal sabkhas or as extensive low barchanoid dune fields, sand sheets and dune ridges. The northern part of the district includes part of the coastal sabkha, which is largely underlain by deflated unconsolidated quartzose aeolian sand and the calcareous Ghayathi Formation aeolianite.Keywords:
Sabkha
Siltstone
Bedrock
Marl
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Summary: The Ojo Caliente Sandstone Member of the Tesuque Formation in the Espanola Basin, New Mexico, consists of a thick sequence of eolian-dune sandstone with rare interdune and clastic sabkha sandstone. The general lack of interdune and clastic sabkha deposits between the eolian dune sandstones suggests that the eolian dune sandstone body resembled the present-day Great Sand Dunes, where a thick pile of dune sand accumulated adjacent to the Sangre de Cristo mountain front, largely above the regional water table. Diagenesi s in the sandstones was dominated by the evolution of non-marine pore waters that produced silica crusts, low-magnesium calcite, iron-rich smectite, and clinoptilolite (a zeolite). Some of the diagenesis may be related to the alteration of chemically unstable volcanic detritus in the sandstones. Calcite cement forms thick layers that follow as well as cut across bedding in the eolian sandstones. These layers would serve as baffles to fluid flow in the sandstones, and have a deleterious effect both for aquifer or oil reservoir development.
Sabkha
Detritus
Micrite
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Strata of the Campbellton Formation, nearly 1 km-thick and known for its diverse fossil assemblage of early plants, arthropods, and fish, can be divided into six facies associations: (1) restricted lacustrine, (2) marginal lacustrine, (3) near-shore lacustrine, (4) coastal-deltaic, (5) sandy to gravelly alluvial plain, and (6) gravelly proximal alluvial environments. Lacustrine deposits with restricted circulation, due to depth or stagnation, are fine-grained with preserved organic material. The marginal lacustrine association consists of massive siltstone and very fine sandstone, interbedded with conglomerate. The latter are interpreted to have shed from older volcanic units forming the basin walls. The near-shore lacustrine association is characterized by rippled sandstone with microbialites. Alluvial strata include interbedded imbricate to nonimbricate conglomerate, trough cross-stratified sandstone, and barren to plant-bearing siltstone. Rare exposures of thickly bedded imbricate to weakly imbricated cobble–boulder conglomerate with sandy plant-bearing lenses are interpreted as products of hyperconcentrated debris flows. In the western belt, a braided-fluvial system had paleocurrents flowing WNW. Coastal-deltaic deposits west of the fluvial outcrops, containing aquatic vertebrates and invertebrates, had paleocurrents flowing ESE, suggesting a confined body of fresh or brackish water. In lower parts of the eastern belt, lacustrine facies are prevalent, representing a large open lake. Alluvial facies dominate upper parts of the formation, representing an eastward-flowing axial braided river system, with proximal alluvium shed transversely from the basin margins. Although most strata have a volcanic provenance, only one outcrop in the lacustrine beds shows evidence of active volcanism during deposition of the Campbellton Formation.
Siltstone
Conglomerate
Outcrop
Cobble
Alluvial fan
Overbank
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Rocks present in the mapped area range from
Early Precambrian (Easement granite) to the most recent
deposits of aeolian sands, alluvium, gravel and calcrete.
Outcrops of Ventersdorp rocks are scarce and
widely scattered. Nevertheless, agglomerates and felsic
lavas of the Makwassie Quartz Porphyry Formation, fanglomerates,
quartzites, volcanic breccias, tuff, mafic massive
lavas and pillow lavas of the Rietgat Formation, quartzites
of the Bothaville Formation and plateau lavas of the Allanridge
formation could be distinguished and studied. The
latter were all grouped together in accordance with the
lithostratigraphic classification of Winter (1976) because
of the proximity to his type area.
Overlying the Ventersdorp Supergroup are shales
and/ or tillite of the Karoo Sequence. Bore holes drilled
to the east (outside) of the mapped area proved that
Dwyka glacial deposits are preserved in pre-Karoo valleys
and/ or valleys excavated through ice movement. Here, the
Dwyka formation consists of tillite, varved shales and
a glacio-fluvial unit, viz. the sandstone-siltstone-shale
unit. In the western sector of the mapped area. Ventersdorp
rocks are directly overlain by black, micaceous shales of
the Ecca Group (Prince Albert Formation). The Whitehill
Formation is probably not present as one continuous layer
over the whole of the mapped area, and only occurs in a
few isolated pockets. As a result the Prince Albert Formation is in most cases overlain by grey shales of the Tierberg formation which grades from shales and mudstones (marine deposits) upwards into rhymic layers of shales and sandstones (deltaic deposits).
The transitional sone between the Ecca Group (Tierberg formation) and the overlying Beaufort group is exposed on the farm Basberg 416. The Beaufort group in this area shows the typical features of a fluvial deposit, viz. course-grained and channel-axis facies which laterally grade into finer grained and thin-bedded channelmarginal facies and levee mudstone deposits on the flood plain of a braided river. Intrusive rocks consist of post-Karoo dolerites (sills and dykes) and the kimberlite intrusions at Rovic Diamond Mines.
Nodular and laminated calcrete deposits are the most abundant calcrete deposits in the area under investigation. Aeolian sands cover large tracts of this area and occur mainly as Aeolian sheet deposits buut also as dunes of variable magnitude.
The Vaal River Gravels and the alluvial sands and silts of the Vet and Sand Rivers are the most significant alluvial deposits present in the mapped area.
Pans of variable shapes and sizes have iriginated through the erosion of paleo river channels by subsequent wind action.
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Outcrop
Breccia
Pillow lava
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Bristol Dry Lake is a 155-sq km fluvial-lacustrine dominated, continental sabkha, or playa, in the Mojave Desert of southeastern California, and is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. The evaporite facies roughly form a bull's-eye pattern with abundnat gypsum and anhydrite surrounding a basin center accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan and sand flat to the center of the playa, reveal (1) crudely bedded, coarse-grained clastics prograding over and interfingering with either (2) wadi (alluvial/eolian) sand and silt, or (3) mud-flat facies of nodular to enterolithic gypsum or anhydrite and blades of gypsum in red-brown silt and clay, followed by (4) saline mud-flat facies f red-brown silt and clay crowded with giant (15 cm diameter), displacive, hopper-shaped crystals of halite, and (5) salt-pan beds of chaotic mud-halite up to 4 m thick in the center of the playa. Deposition of terrigenous clastics was by fluvial-sheetflood processes around the toes of alluvial fans, fluvial flow through very shallow rills and suspension settling in the mud-flat environments. Much of the sediment is reworked by eolian processes. Evaporites are precipitated at or just below the sabkha surface from discharging brines. Lithofacies of this modern continental sabkha are nearly identical to those comprising the Middle-Upper Permian evaporites of the Texas and Oklahoma panhandles, and they are excellent process analogs for ancient facies analysis. End_of_Article - Last_Page 934------------
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he Island of Hvar is located along the NE coast of the Central Adriatic, Croatia. Along its W– E oriented longer axis it stretches for almost 70 km, while it is approximately 5 km (only in the western part up to 10 km) wide. It is composed mostly of Cretaceous carbonate deposits (stratigraphic range Neocomian– Maastrichtian), originating from the ancient Adriatic Carbonate Platform, one of the largest and best preserved Perimediterranean carbonate platforms [1]. The structure of the Island of Hvar is characterized by asymmetrical anticline composed of Cretaceous deposits (limestones and dolomites) which is, along the southern margin of the island, in the tectonic contact with the youngermost Cretaceous limestones and Eocene deposits (foraminifera limestones, transitional marls, and flysch). Structure of the island (oriented W– E) differs from the general regional strike (so called Dinaric strike – NW– SE). Major outcrops of the Pleistocene deposits [2] are situated in the western, widest part of the island where U-shaped valley developed. Studied deposits crop out at the southern side of the valley. The outcrop is 35 m long and up to 12 m high, and is composed of sands and sandstones, and subordinate breccia. Lower part is dominated by a planar cross-bedded unit composed of sand and sandstone. Its thickness is up to 2.5 m, varying laterally because of erosion. Cross-beds are steep (20– 31°), mostly tangential, sigmoidal at places. Reactivation surfaces are frequent within the unit. Breccias occur as irregular lens-shaped bodies with scoured bases at the level of the upper bedding plane of the cross-bedded unit. Medial part is more complex. The sands and sandstones are represented by smaller units showing generally tangential cross-bedding of lower angle, and trough cross-bedding. Breccia lens-shaped bodies are more frequent, and occur both within cross-bedded deposits and at the level of the upper bedding planes of the cross-bedded units. Some isolated carbonate clasts can be found within sands. In the upper part of the outcrop low-angle small-scale cross-bedded units prevail, and scouring is rare. The cross-bedding in whole cross-section documents general palaeotransport to the west. Breccia lens-shaped bodies are of different geometry, from one-clast thick lenses to 70 cm deep irregular erosional bodies, and are up to 3 m wide. Breccias are clast- to matrix-supported, consisting of carbonate fragments of local origin with sandy matrix. The grain-size distribution is bimodal. Grain-size of the breccia fragments extremely varies from granule to boulders up to few dm long. The largest fragments are usually related to deepest erosion, i.e. they occur in more narrow but deeper lenses, where content of sandy matrix is low. Some bodies show channel-fill. Sands and sandstones are mostly fine- to medium-grained, well sorted, rich in carbonate content (70% in average). The sandstones, determined as calclithites, mostly consist of limestone extraclasts. Rock fragments (cherts, quartz– sericite schists, quartzite) and quartz dominate in siliciclastic component of the sandstones. The sands, both in cross-bedded units and in breccia lenses, are of the same composition as the sandstones. Amphiboles, pyroxene, garnets and epidote are their most abundant translucent heavy minerals. The dominance of chemically unstable rock fragments and heavy minerals in the composition of clastic material of the Island of Hvar suggests its mineralogical and petrological immaturity. This immaturity is related to the intense mechanical and less intense chemical weathering of the source rocks. The composition of the material indicates their provenance from carbonate and siliciclastic sedimentary and metamorphic rocks. The structural maturity of material is probably linked to the long distance transport and/or to the transport by wind. The cross-bedded sand and sandstone units are of aeolian origin, and represent dunes. The dunes migrated by influence of strong eastern winds generated by cold and arid climate. The sand source area were probably river flood-plains within Dinarides which are composed of carbonate, siliciclastic, magmatic and metamorphic rocks. The breccia lens-shaped bodies represent alluvial deposits of local origin, which accumulated at the foot of a steep hill slopes possible as a consequence of snow melting during short less cold and humid periods. The occurrence of heavy minerals in its sandy matrix can be explained by erosion and redeposition of aeolian deposits. Isolated limestone clasts within cross-bedded units are probably a consequence of subaerial rock-fall.
Outcrop
Marl
Flysch
Carbonate platform
Breccia
Onlap
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Red beds
Conglomerate
Alluvial fan
Halite
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Lateral and vertical facies variations within the predominantly eolian upper member of the Minnelusa Formation control both the regional reservoir distribution and the localization of oil-producing trends. Sands sourced by northeasterly trade winds were deposited in a land area bounded on the west by the Lusk embayment, which was a shallow, restricted extension of the Permo-Pennsylvanian sea. This embayment was present throughout Minnelusa deposition, and was located in the western portion of the present-day Powder River basin. Another extension of the epeiric sea, located in western South Dakota, formed the eastern boundary of the land area. In the northern part of this area, an inland sand-sea developed; in the southern part, the sand supply was less and isolated barchan dunes migrated over a coastal sabkha. Dune sandstones are bounded laterally by predominantly sandy interdune deposits in the north and by coastal interdune deposits, including sandstone, dolomite, and anhydrite, in th south. Major marine transgressions deposited laterally extensive dolomites that separate the dune sandstones. Interdune deposits constitute permeability barriers adjacent to dune sandstones. The dune sandstones, which can be of excellent reservoir quality, were subjected to early cementation by anhydrite. Later dissolution of the anhydrite cement, facilitated by good to excellent sorting and possibly enhanced by hydrocarbon migration, led to development of significant secondary porosity. Interdune sandstones are less well sorted and so did not develop good secondary porosity. Interdune carbonates and evaporites have virtually no permeability. The coastal interdune deposits in the southern part of the region, therefore, form more effective lateral permeability barriers than do the sand-dominated interdune deposits in the north. End_of_Article - Last_Page 852------------
Oil Production
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The Red Cave Formation (Permian, Leonard Series) in the Texas Panhandle consists of cyclic, red-bed clastic and carbonate-evaporite members that were deposited in an extensive coastal sabkha, desert wadi plain, and a carbonate inner shelf which bordered the northern Midland basin. Evaporite members were deposited in carbonate-evaporite crustal sabkhas and clastic members were deposited in mud-rich coastal to continental sabkhas. North to south, red-bed wadi-plain facies pass into coastal sabkha facies and inner-shelf dolomite facies. In a Randall County core, vertical sequences commonly include slightly fossiliferous, faintly laminated to burrowed dolomitic mudstone and pellet wackestone overlain by cross-laminated oolitic or pellet packstone to grainstone, followed by algal-laminated dolomitic mudstone and nodular anhydrite in dolomite matrix. A progradational carbonate shoreline is inferred, with supratidal or sabkha evaporite to intertidal algal-mat and sand-flat environments passing seaward into a shallow, muddy subtidal inner shelf. Mud-rich sabkha sequences culminate with red to green mudstone and anhydrite above shoreline carbonates. Carbonate and evaporite facies pinch out generally toward the northwe t and northeast into wadi-plain red beds. These facies include ripple-drift cross-laminated siltstone and sandstone deposited in braided fluvial channels, adhesion-rippled siltstone, and red to green mudstone deposited in mud-flat and interchannel environments. Desiccation features, intraclasts, root zones, and paleosol horizons attest to subaerial exposure and probable nonmarine conditions. Large-scale cyclicity of red-bed clastic and carbonate-evaporite members probably was controlled by the relative supply or fluctuating input of clastics to sabkhas by way of fluvial systems rather than by absolute sea-level changes. Partial modern analogs to Red Cave sabkha depositional models are the coastal mud flats and alluvial fans in the northwestern Gulf of California, tidal flats and an ephemeral stream delta (Wooramel delta) in Gladstone Embayment, Shark Bay, Australia, and the Trucial Coast sabkhas in the Persian Gulf. Each setting has certain facets that are remarkably similar to intepreted paleoenvironments and lithofacies of the Red Cave Formation. End_of_Article - Last_Page 761------------
Sabkha
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ABSTRACT Bristol Dry Lake, a 155 km 2 continental‐sabkha playa basin in the Mojave Desert of south eastern California, is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. Evaporite facies conform approximately to a bull's eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan to the centre of the playa, reveal: (1) crudely‐bedded, alluvial fan clastics interfingering with (2) playa‐margin sand flat and wadi sand and silt, followed by (3) gypsum, anhydrite, chaotic mud halite, and clay of the saline mud flat, and (4) salt‐pan halite beds. Terrigenous clastics were deposited in Bristol Dry Lake by sheetflow and by suspension settling from ponded floodwater. Some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin. Thin nodular‐like beds of anhydrite and several types of gypsum occur across most of the playa. Giant hopper‐shaped halite cubes are suspended in saline mud flat facies, suggesting that they grew displacively in brine soaked sediment just below the surface. Thick beds (4 m) of halite, in the playa centre, may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods, intense evaporation and brine‐level lowering, and capillary discharge of brines. The stratigraphy in the playa centre is cyclic. An ideal cycle consists of: (1) chaotic mud halite at the base overlain by (2) green to red clay with abundant, giant hoppers, and at the top (3) red clay, gypsum, and anhydrite with flaser‐ to wavy‐bedded sand and silt. This type of cycle probably records a gradual progradation of mud‐flat facies over salt pans. Bristol Dry Lake sediments are nearly identical to some of the Permian evaporites of the Permian Basin region, U.S.A. and they can serve as modern analogues for ancient‐sabkha facies analysis.
Halite
Anhydrite
Sabkha
Terrigenous sediment
Salt pan
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