The distribution pattern of the microfacies in G. Maghara reflects transgression from tidal environments of the inner ramp at the base to relatively deep-water environments of the outer ramp at the top of the succession (Table 2.1). Thirty different microfacies types have been recognized and grouped into eight depositional environments ranging from deltaic, tidal, restricted lagoon, open lagoon, and shoal, to middle ramp, slope, and outer ramp.
Sedimentation took place on a carbonate ramp which physiographically varied from homoclinal to distally steepened.
The inner ramp facies are widespread in both the Mahl and Bir Maghara formations. They include tidal deposits of sandy fine-grained mudstones and dolomitized algal wackestones with admixture of quartz grains. Phosphatic oo-grainstones, sandstones and bio-oo-packstones probably correspond to sandy shoals and banks of the inner ramp.
Protected lagoonal settings of the inner ramp are characterized by peloidal packstones.
Sediments formed in open-marine settings of the inner ramp are bio-packstones. Sediments of the mid-ramp are represented by bioturbated, glauconitic foraminifera-ostracod onco-wacke- to packstones. Echinoderm grainstones represent the slope facies and constitute the Masajid Formation. Outer ramp deposits are represented by spiculite wackestones and bio-mud- to wackestones. Mid-outer ramp sediments are typical of the Kehailia and Arousiah formations. The following descriptions provide a summary of these facies.
Table 2.1. Description and interpretation of the main facies associations.
Microfacies Components/features Environme
Bio-wackestone Thin-bedded; bioclasts increase upward and include foraminifers, bivalves, nt sponge spicules, Neuropora, brachiopods, ferruginous nerineid gastropods, radiolarians, filaments, and algae. Brachiopod and gastropods shells
common, well preserved. Lower and upper contacts sharp. Low-energy outer ramp
Wacke-/mudstone Poorly laminated dark grey bioturbated mudstone with dispersed sponge spicules and ammonite fragments. Concentrations of Pholadomya found in the lower part. Other fossils rare. Lower and upper contacts gradational.
Shale Fine-laminated to fissile, yellow to dark green glauconitic, organic-rich, with tiny pyritized ammonites. Authigenic minerals (glauconite and pyrite) abundant. Benthic fauna completely absent.
Bio-grain- to
rudstone Yellow, thick-bedded (2-8 m), coarse-grained calcarenites. Ferruginous and siliceous concretions abundant. Bioclasts include crinoids, echinoids, and silicified molluscs. Intraclasts of minor importance. Cidaroid spines
common. Lower bedding plane sharp, upper contact gradational. High-energy
upper slope
Oo-bio-grainstone Graded, medium- to thick-bedded grainstones alternating with cidaroid-rich marls. Poorly sorted alternations of ooids and bioclasts; dominant bioclasts include foraminifers, echinoids, coral fragments, crinoid fragments, sponges; intraclasts. Lower bedding plane erosional.
Oo-grainstone Graded, medium- to thick-bedded. Well sorted ooids with minor oncoids and numerous cidaroid spines. Lower bedding plane erosional.
Microfacies Components/features Environme Bio-onco- nt
floatstone Highly bioturbated, medium-bedded; ferruginous and siliceous concretions abundant. Fossiliferous, light grey, alternating with marl. Oncoids large and irregular. Encrustation high, meiofaunal bioturbation and micritic envelopes occur. Bioclasts dominate and include algae, echinoids,
gastropods, bivalves, foraminifers, corals, and sponges. Low- to moderate - energy middle
ramp
Onco-biofloatstone Highly bioturbated, fossiliferous, light-grey, medium-bedded; ferruginous and siliceous concretions abundant, coral patches occur; poorly-sorted;
meiofaunal bioturbation common; with nerineids, Goniocylindrites, bivalves, foraminifers, corals, ostracods, and some oncoids. Lower and upper contacts gradational.
Onco-wackestone Bioturbated, grey, medium-grained, medium-bedded. Apart from oncoids bioclasts dominate and include gastropods, foraminifers, sponge spicules, echinoids, codiacean green algae, “Girvanella”, and some corals. Some grains are coated. The mollusc fragments are silicified and encrusted.
Bio-onco-rudstone Thin-bedded limestone alternating with sandstone. Oncoids are the most common components; followed by moderately sorted bioclasts (echinoids, Bio-packstone Thin-bedded; with molluscs, echinoid, crinoid, and coral fragments, sponge
spicules, and calcispheres. Micritic envelopes common.
Bio-grainstone Thin-bedded, chamositic-phosphatic with shell concentrations; intraclasts, quartz grains, and grain aggregates occur. Oysters abundant. Shells encrusted and bored.
Bio-oo-grainstone Thin, graded, chamositic beds with well sorted concentric ooids. Peloids, microbial coated grains, oncoids and intraclasts also occur. Some bioclasts of bivalves and gastropods. Bioturbation abundant.
Fossiliferous
sandstone Fossiliferous, well sorted, ferruginous. Bioclasts common. Echinoids and gastropods dominate.
Pel-bio-packstone Bioturbated, thick bedded packstone. Peloids common, bioclasts less common (brachiopods, bivalves, crinoids, sponges, ammonites, and
echinoid spines. Pyrite grains occur. Low to
Pel-bio-wackestone Bioturbated, greenish brown, medium- to thick-bedded, grey limestone, with poorly sorted peloids, few bioclasts, intraclasts, and oncoids. Bioclasts include corals, gastropods, miliolid foraminifers, serpulids, and sponges.
Large terebratulid brachiopods abundant.
Pel-packstone Poorly sorted peloids, recrystallized bioclasts, sponge spicules. Few oncoids and ooids also present. Lower and upper contacts gradational.
Encrusted corals debris common.
Calcareous
sandstone Low-angle planar cross-bedded, poorly-sorted, ferruginous, arkosic with small bivalve fragments. Ichnotaxa include Thalassinoides and
Diplocraterion. Lower and upper contacts erosional. High-energy shoreface Onco-pack- to
rudstone Well sorted oncoids, some bioclasts including sponge spicules, gastropod shell fragments, corals, and Neuropora. Micrite increases upward.
Onco-grainstone Medium-bedded with erosional base. Well sorted, with bioclasts, ooids, encrusted shells, algae, sponge spicules, and coated gastropod shell fragments.
Intra-wackestone Lower bedding plane sharp, upper one slightly bioturbated. With bands rich in silt grains and mud. Few fibrous gypsum lenses occur. A few siliciclastic
particles, including quartz grains, occur as does dispersed organic matter. Low-energy intertidal Sand-silt-clay
interbeds Wavy to flaser laminated alternations of fine- to medium-grained sand, silt and clay; plant remains occur.
Non-fossiliferous sandstone
Low-angle trough cross-bedded arkosic medium- to coarse-grained or moderately bioturbated and immature, yellowish brown, fine to medium-grained, subangular to subrounded with calcareous cement; moderately sorted with some rock fragments and silica overgrowth. Dolomitic in some parts. Wood fragments and plant remains occur.
sandstone Thin, ripple-bedded, siltstone and fine-grained sandstone with sharp
undulating top. Plant remains and trace fossils occur. No fossils observed. Moderate -energy
delta front Shale Clay and/or siltstone, bed thickness varying from 0.9 to 7 m. Silt grains
Low-Microfacies Components/features Environme increase in size upwards. Upper contacts gradational, lower ones sharp, nt
undulating. Oysters, Protocardia, and Grammatodon occur. energy prodelta
3.2.1 Inner ramp
The homoclinal ramp model (Burchette and Wright, 1992) distinguishes between inner, middle and outer ramp. Inner ramp sediments are deposited in an agitated environment above the fair-weather wave-base (FWWB). It ranges from the shallow subtidal to the intertidal zone. Lagoonal settings sheltered by barriers may develop (Tucker, 1985). In mixed siliciclastic-carbonate rocks of the inner ramp, sandy allochemic limestones (composed of ooids, bioclasts, and lithoclasts) are the most common rock types. Small patch reefs may also occur.
3.2.2 Middle ramp
Middle ramp deposits consist mainly of packstone or grainstones deposited above the storm wave-base (SWB). Sedimentation is therefore highly influenced by variable storm intensity (Burchette and Wright, 1992; Flügel, 2004). Autochthonous and allochthonous shells occur.
3.2.3 Outer ramp
The outer ramp is dominated by thin, laminated beds of terrigenous mud, with parautochthonous and autochthonous shells. Skeletal wackestones dominate, some lime mud formed in situ (Tucker, 1985). The presence of ammonites and belemnites indicates a comparatively distal environment. The absence of primary sedimentary structures produced by currents or waves suggests a low-energy environment probably located below the SWB.
3.2.4 Slope
The distally steepened ramp of G. Maghara tectonically developed from a previous homoclinal ramp. The sediments of the slope are largely autochthonous to para-autochthonous. Slumping and breccia deposits are of minor importance. Bio-oo-grainstones and rudstones characterize the slope. It resembles the ramp model proposed for the Upper Triassic shelf and platform margins in the Northern Calcareous Alps by Stanton and Flügel (1995).
3.2.5 Delta
The Early Bathonian Safa Formation represents an upward-coarsening siliciclastic succession typical of a deltaic setting (Fig. 3.4D-F). It comprises prodelta (shale, silt, and clay), delta front (rippled sandstone) and distributary channel (low-angle trough cross-bedded sandstone) subenvironments. The total thickness of these deltaic sediments is 200 m. The occurrence of elongated tidal bars and periodic abandonment surfaces in addition to numerous channels indicate the tide-dominated delta type (the action of tidal currents significantly overprints that of channel flow; see Renaud and Kuenzer, 2012). Tide-dominated deltas are directly connected to the sea via a series of distributary channels that are typically flanked by low-lying vegetated floodplains and swamp areas (the source of coal deposits at Gebel Maghara).