Paleoenvironmental setting and facies analysis of the Middle Miocene carbonate platform in north Western Desert, Egypt

The Middle Miocene rocks in the northern part of the Western Desert of Egypt are dominated by carbonate rocks that constitute the Marmarica Formation. To determine the depositional environments and sequence architecture of the Marmarica Formation, three sections were described and sampled in detail. These sections from west to east are; Girba, Naqb El‐Migahhiz, and Gabal Umm Hiyus. The Marmarica Formation in the Siwa area includes three members, from bottom to top: the Oasis, Siwa Escarpment, and El Diffa Plateau members. The microfacies of the Marmarica Formation comprise abundant mud‐supported microfacies with minor grain‐supported microfacies. Sedimentation during the Middle Miocene took place on a homoclinal ramp. Dolomitization is the most prevalent diagenetic process that affected carbonate sediments of the Marmarica Formation. The Marmarica Formation comprises two sequence boundaries. The first sequence boundary (SB1) is demarcated by the presence of Thalassinoides ichnofacies at the boundary between the Oasis and Siwa Escarpment members in all the studied sections. It is considered a type 1 sequence boundary. The second sequence boundary (SB2) is represented by an erosional surface, which is recorded only in Girba section. The SB2 was traced at both Naqb El‐Migahhiz and Umm Hiyus sections by the occurrence of burrows that relate to the Glossifungites ichnofacies. The latter boundary is considered type 2. Three third‐order depositional sequences have been recognized in the Middle Miocene Marmarica Formation. Therefore, the Marmarica Formation at Siwa Oasis exhibits relative sea‐level oscillation. During the Middle Miocene, the prevailing warm climate caused transgression and deposition of carbonates along the marine shelf surrounding western and northern Africa.

Here we present the stratigraphic architecture and carbonate platform evolution of the Middle Permian (Capitanian) succession (Wargal Formation) of Trans‐Indus Ranges, central Pakistan. The exposed carbonate sequence consists of thin‐ to thick‐bedded bioclastic, nodular, fine‐grained, and sandy limestone. Based on detailed petrographic studies, seven microfacies assemblages have been recognized, suggesting diverse depositional environments including mudflats, lagoon, sand shoal, and middle shelf. The identified age‐diagnostic larger benthic foraminiferal species enabled to assign a Middle Permian (Capitanian) age to the studied section of the Wargal Formation. Based on the vertical arrangements of depositional environments in time, three Transgressive Systems Tracts (TSTs), two Regressive Systems Tracts (RSTs), and 11 high‐resolution fourth/fifth‐order parasequences were delineated. The depositional and sequence stratigraphic analyses of the studied section inferred evolution of the Middle Permian carbonate platform in three stages: (i) marginal marine facies development in the lower part followed by (ii) transitional stage in the middle part, and (iii) finally, aggrading shallow marine facies stage in the upper part. The evolution of carbonate platform can be linked with the cessation of major rifting followed by thermal cooling and establishment of marine Tethyan settings at the margin of the rift flank basin. The diverse facies variation in the second stage of platform evolution argues strongly for the local intra‐plate stresses. Based on petrographic, SEM, and EDS studies, it is concluded that the studied section of Wargal Formation carries porosity values from a range of 2–8% with an average of 5% and can be called a low to moderate hydrocarbon reservoir in the study area.

The Ginsburg model of carbonate accumulation is an often-quoted mechanism for generating so-called autocycles. It is shown that the model does not represent a self-oscillating system; oscillations can only be generated if at least two critical parameters controlling sedimentation are introduced. The Ginsburg model is a conceptual model which tries to explain the behavior of some carbonate shelves that undergo continuous tectonic subsidence and that carry on their surface a very active carbonate factory. Unfortunately the original model has only been published in abstract form—the relevant part of which is quoted here in full (Ginsburg, 1971, p. 340): . . . "The Florida Bay lagoon and the tidal flats of the Bahamas and Persian Gulf are traps for line sediment produced on the large adjacent open platforms or shelves. The extensive source areas produce carbonate mud by precipitation and by the disintegration of organic skeletons. The carbonate mud moves shoreward by wind-driven, tidal or estuarinelike circulation, and deposition is accelerated and stabilized by marine plants and animals. Because the open marine source areas are many times larger than the nearshore traps, seaward progradation of the wedge of sediments is inevitable. This seaward progradation gives a regressive cycle from open marine shelf or platform to supratidal flat. As the shoreline progrades seaward the size of the open marine source area decreases; eventually reduced production of mud no longer exceeds slow continuous subsidence and a new transgression begins. When the source area expands so that production again exceeds subsidence a new regressive cycle starts.". . . The author is very grateful to Dr. Ginsburg for supplying some additional information that is not obvious from the abstract. The subsidence must be differential and a broad, open shelf that gradually tilts seaward is visualized. All of the sediment produced on the shelf is transported shoreward, where it accumulates as a wedge-shaped deposit that builds into a tidal bank. A further analysis of the model is interesting for two reasons. First, the model has been and still is seriously suggested as a possible mechanism to explain cyclicity on carbonate platforms (see Goldhammer et al, 1987, for references).

This study focuses on the effect of diagenetic processes on the petrophysical characteristics of the Miocene rocks exposed east of the Qattara Depression, north Western Desert, Egypt. Several techniques were applied on the collected rock samples in order to determine their mineralogic composition and the diagenetic processes they have undergone. The petrophysical analyses are conducted on horizontal plugs representing the Miocene Formations in the study area. They mainly include porosity, permeability, and density. Petrographic analysis revealed that the Moghra Formation is composed mainly of quartzarenites with few shale and limestone intercalations. The Marmarica Formation, on the other hand, is composed mainly of sandy dolomicrite, sandy biodolomicrite, and sandy biomicrite facies. The main diagenetic processes encountered are neomorphism, dolomitization, dissolution, cementation, compaction, and replacement. Values of porosity, permeability, grain, and bulk densities for the studied plugs derived from the Moghra Formation range from 14.7 to 27.8 %, from 0.01 to 35.39 mD, from 2.51 to 2.79, and from 2.01 to 2.32 g/cm3, respectively, while they range from 2.9 to 40.8 %, from 0.002 to 14739.15 mD, from 2.67 to 2.8 g/cm3, and from 1.62 to 2.65 g/cm3 for the samples of Marmarica Formation. Both petrographical and petrophysical studies revealed primary and secondary origins of the sample porosity and permeability and that the studied sandstones can be considered as good hydrocarbon reservoirs. In addition, the studied carbonate rocks are characterized by high effective porosity and permeability due to the secondary enhancement through the dissolution of fossils and other components implying that their corresponding subsurface occurrences represent good reservoir rocks.

Hummocky cross-stratification from the Lower Jurassic Hasle Formation of Bornholm, Denmark

Microfacies studies were carried out on the Middle Miocene Marmarica Formation exposed at the Gabal Western Bahi El-Din and Gabal El-Najdeen, the Siwa Oasis, northwestern Desert (Egypt). It was distinguished into the lower, middle, and upper members. Eleven microfacies types were recognized, which include skeletal lime-mudstone, dolomitic lime-mudstone, intraclastic wackestone, bryozoan wackestone, foraminiferal wackestone, foraminiferal bryozoan packstone, glauconitic molluscan packstone, molluscan intraclastic packstone, pelletal peloidal skeletal packstone, dolostones, and claystone microfacies. This formation includes several types of emergence- meter-scale cycles (shallowing-upward). Field observations and petrographic analyses revealed that these cycles consist of pure carbonates and mixed siliciclastic carbonates. These cycles consist of four types of gradual cycles and six types of non-gradual cycles. The gradual emergence cycles indicate a balance between the rate of subsidence, sea level oscillations, and sedimentation rate. The non-gradual cycles indicate an irregular balance between sedimentation rate and subsidence rate. The non-gradual cycles denote high-frequency sea level variation and/or short-term sea level oscillations, which are associated with high carbonate formation. The depositional environments of the Marmarica Formation are restricted to lagoonal at the base, followed upward to open marine conditions. Both environments most probably characterize the platform setting.

Calcareous, quartzose and organic-rich (peaty) sediments accumulating in two modern mangrove estuarine swamps, the Ten Thousand Islands and the Whitewater Bay areas of southwestern Florida, were studied to derive criteria that would aid in recognizing similar ancient coastal swamp deposits. Both areas are at the seaward margin of the Everglades and are associated with an imposing coastal mangrove forest. The sediments forming in these two coastal swamps are modern analogues of ancient limestone, quartzite, and coal beds, and the coastal mangrove forest is the modern counterpart of ancient coal forests of cratonic and miogeosynclinal areas. Evaluation of results yields three principal criteria which may be useful in distinguishing ancient coastal swamp deposits. 1. Organic matter and carbon/nitrogen ratio: The content of organic matter and the ratio of organic carbon to nitrogen in surface sediments (upper 0.25-0.50 foot) along the southwestern Florida coast increases toward land on the open marine shelf and attain their highest values (organic matter ranging from 6 to more than 50 per cent and C/N ratio ranging from 17 to 37 per cent) within the coastal swamps. The increase in organic matter and C/N ratio in a shoreward direction is due to a progressive increase in the amount of organic matter contributed to surface sediments by terrestrial plants (mangrove trees) compared with that supplied by plankton and benthos. Fossil coastal swamp deposits, therefore, may be recognized by their exceptionally high organic matter content and C/N ratio with respect to laterally adjacent sediments. 2. Faunal assemblages: The areal distribution of molluscan faunal assemblages in the vicinity of the coastal swamps of southwestern Florida suggests that biofacies of ancient coastal swamp deposits would trend parallel with the coastline and delineate a direction of decreasing average water salinity toward the land area supplying run-off waters to the coastal swamps. 3. Grain size of carbonate and detrital minerals: Calcareous particles and detrital quartz along the southwestern Florida coast decrease in grain size from the open marine shelf toward and into the estuarine mangrove swamps. Hence, with support by criteria 1 ind 2, fossil coastal swamp sediments may be differentiated by their tendency to be noticeably finer-grained than adjacent sediments. End_of_Article - Last_Page 278------------

Investigation of well-cutting samples from five drill holes penetrating the Lower Tuscaloosa Formation in the Sand Hill (Greene County), Stewart (Pearl River County), and McComb (Pike County) Fields revealed foraminifera from the early Late Cretaceous (Late Cenomanian) Rotalipora cushmani-greenhornensis biozone, and bivalve shells indicative of open marine shelf conditions. Samples from the Sand Hill Field contained the foraminifera Hedbergella delrioensis and Loeblichella hessi and bivalve shells. Samples from the Stewart Field contained the foraminifera Heterohelix moremani and Rotalipora appenninica and bivalve shells. Samples from the McComb Field contained only poorly-preserved, recrystallized specimens similar in general form, chamber arrangement, and size to Heterohelix moremani and Rotalipora greenhornensis. Age-correlation of foraminiferal species found in all three locations agrees with an assignment of all fossiliferous samples to the Rotalipora cushmani-greenhornensis biozone. The presence of these Late Cenomanian (Woodbinian-Eaglefordian) foraminifera with open marine shelf bivalve remains suggests that marine conditions prevailed during the deposition of the Lower Tuscaloosa Formation across southern Mississippi, and correlate with similar open-marine shelf facies in the South Carlton Field in Clarke County, southwestern Alabama.

Sedimentary evolution of the Nakdong River deposits on the Korea Strait shelf since the Last Glacial Maximum

Palynological investigations of samples collected from the Abu Roash Formation, Faghur Hj5-1 well, north Western Desert, Egypt show a low diversity in palynomorph assemblage. This assemblage is mainly dominated by a clear proliferation of Pediastrum (and other allied algal forms, e.g., Scenedesmus and Botryococcus) which today lives exclusively in freshwater. Such a prominent record within the current marine deposits could be considered a good biostratigraphic datum in the Upper Cretaceous (Turonian) period rather than an ecologic event in the north Western Desert, Egypt. The influence of freshwater input on the studied deposits is proven by the presence of heavy minerals including kyanite, zircon, staurolite, and amphiboles encountered in siliciclastic sediments. Most of these minerals are sub-rounded to rounded so they were derived probably from sedimentary rocks. In the studied succession, the presence of Pediastrum reflects sea level fall (i.e., lowstand systems tracts). The occurrence of Pediastrum and other algae in such marine deposits reflects the predominant deposition of fluviatile sediments related to the discharge of rivers into shelf seas.

Conventional sequence-stratigraphic concepts relate the large-scale stratal architecture of sediment bodies to changes in relative sea level. This paper evaluates the relationship between stratal geometry and sea-level stand, based on a study of large, semicontinuous outcrops of Cretaceous carbonate platform strata in the Vercors (southeastern France). Multiple lines of evidence for sea-level change are combined, including stratal geometry, detailed quantitative microfacies analyses, and diagenetic patterns at platform-top hardground surfaces. The studied outcrops of the Cirque d'Archiane show two main prograding platform tongues, both over 100 m thick. The stratal geometries at the boundary between these platform tongues, including an apparent pinchout of a wedge of slope sediments, suggest the presence of a major lowstand unconformity. However, this stratal boundary does not coincide with the horizon containing the most extensive meteoric alteration. Furthermore, detailed platform-to-basin correlation shows that the wedge of slope sediments is not basin-restricted, but makes a thin drape over the platform top. The sedimentologic and diagenetic evidence suggest incipient drowning of the platform at the boundary between the two main platform tongues, preceded by a minor exposure event only. Internally, the main platform tongues consist of smaller (10-30 m thick) units that prograde towards the basin and aggrade on the platform top, and which are interpreted as stacked highstand wedges. These wedges are usually topped by hardground surfaces with minor evidence for subaerial exposure. The platform-top horizon with the most extensive subaerial diagenesis and erosion correlates with a distinct but relatively thin unit of lithoclastic debris on the slope. A pronounced scour in the clinoforms of the Vercors does not correlate with a major exposure surface on the platform top. This study shows that stratal geometries alone are a rather ambiguous guide to sea-level history. Without the accompanying sedimentologic and diagenetic evidence for sea-level change, most of the stratal architecture of the Vercors platform can be explained by either changes in accommodation or changes in carbonate production.

Seawater 87Sr/86Sr ratios along continental margins: Patterns and processes in open and restricted shelf domains

Updip sequence development on a wave- and current-dominated, mixed carbonate-siliciclastic continental shelf: Paleogene, North Carolina, eastern U.S.A.

Integrated palynological, organic geochemical, and sequence stratigraphic analyses of the middle to upper Cenomanian hydrocarbon reservoir/source Abu Roash “G” Member: A depositional model in northwestern Egypt

The sequence development and evolution of Lower Carboniferous strata in the Marsel block of the Chu-Sarysu Basin, Kazakhstan have been established through detailed sedimentological analysis of cores, petrophysical data, and new seismic interpretations. Four depositional sequences and eighteen parasequences were classified in the Lower Carboniferous strata. Each depositional sequence is composed of a transgressive systems tract (TST) and highstand systems tract (HST), roughly coinciding, respectively, with the following four stratigraphic intervals (1) lower Tournaisian (C1t) through lower Visean (C1v-1) strata; (2) middle Visean (C1v-2) strata; (3) upper Visean (C1v-3) strata; and (4) Serpukhovian (C1sr) strata. Parasequences are bounded by an instantaneous drowned punctuated surface (C1t and C1v) or instantaneous exposed punctuated surface (C1sr). In addition, petrographic studies have led to the recognition of 20 microfacies, distributed on the major depositional facies zones of the outer ramp (Mf1, Mf6, Mf20), middle ramp (Mf3–5, Mf8–13), and inner ramp (Mf2, Mf4, Mf7, Mf9, Mf12, Mf14–19). They are grouped into seven facies associations corresponding to widespread deposits of tidal flats (MA3, MA5), shallow shoals (MA6), and shallow lagoons (MA4) in the inner ramp; deposits of restricted lagoons (MA1) and local patch shoals (MA7) in the middle ramp and a relatively small outer ramp deposits (MA2). SQSQ All the microfacies and their associations are grouped into the following 6 major facies belts: outer ramp; restricted lagoon, and patch shoals in the middle ramp; and shallow shoals, restricted lagoon, subtidal to intertidal and supratidal in the inner ramp. The interpretation of these facies belts suggests a shallow warm-water tropical-subtropical depositional environment on a passive continental margin ramp. Facies correlation between wells in sequence framework suggests that the strata may be divided into four sequences (SQ1, SQ2, SQ3, SQ4). SQ1 and SQ2 are dominated by lagoon facies and outer ramp facies in the northwest area in the TST, and tidal flat facies in the HST; SQ3 is characterized by restricted to semi-restricted lagoon facies in the TST and local patch shoal facies and mixed tidal flat facies in the HST; SQ4 is marked by restricted lagoon facies in the TST and evaporitic lagoon, tidal flat, and shallow shoal facies in the HST. The research results can be used for investigating the development and distribution of potential reservoirs in the Lower Carboniferous strata of the Chu-Sarysu Basin; the reservoirs in the HST of late Visean and Serpukhovian in the northern and southeastern part of the platform interiors may be of premium quality.