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Abstract

The Simsima Formation (lower Upper Maastrichtian) is a shallow–water carbonate that crops out in eastern United Arab Emirates along the northwestern border of the Oman Mountains. Its thickness ranges in this region from 30 to 80 m. From detailed sedimentological and petrographical analysis, various microfabric types and eight sedimentary fades are recognized in the Simsima Formation. These microfacies are arranged according to their frequency and collected into three groups as follows: packstone group (orbitoidal packstone, bioclastic packstone, algal packstone, and plank tonic foraminiferal packstone); boundstone group (milleporid, coral, and algae), and dolostone, which consists mainly of dolomitized limestone. The Simsima Formation in this study can be subdivided into two new members, called here the lower member and the upper member, each with a distinct character and considerable vertical and horizontal distribution.

Introduction

The Late Cretaceous was one of the most significant periods in the geologic history of the Arabian region. It was a time when major tectonic events affected the Arabian Plate. Subduction at the Zagros geosuture and obduction at the Oman line, together with the consequent tectonic events, left their mark on the region (Murris, 1980). These Late Cretaceous tectonic movements were associated with reactivation of older structures and differential mobilization of ancient sediment deposits.

In the vicinity of the present-day Oman Mountains there is evidence for an oceanic fore-deep basin, which existed during Campanian time. This was filled by the Hawasina sediments and covered by emplacement of the Semail ophiolite (allochthonous complex) (Alsharhan and Nairn, 1990, 1997; Noweir et al., 1998). The ophiolite nappes are overlain, with a marked regional unconformity, by siliciclastic sediments deposited in continental-margin to shallow-marine settings. These, in turn, are overlain with local minor unconformity by the shallow-marine carbonates of the Simsima Formation (Fig. 1). The Simsima Formation in this region is uncomformably overlain by the Muthaymimah Formation (Nolan et al., 1990 and Skelton et al., 1990). Though faulted at many localites, this unconformity is visible on Jebel Thanais (Skelton et al., 1990). The Muthaymimah was originally dated as Paleocene to Eocene by Nolan et al. (1990), but in Sayh Muthaymimah (SE of Buraymi) it ranges from the latest Maastrichtian to the Paleocene. From planktonic foraminifers, they recognize the succession of zones across the Maastrichtian-Danian boundary in its basal part. Both the allochthonous complex and the autochthonous strata (Qahlah and Simsima formations) are exposed along the Oman Mountains as a result of post-Miocene uplift. Other autochthonous strata crop out in peripheral jebels around the mountain chain (Glennie et al., 1974; Skelton et al., 1990; Alsharhan and Nasr, 1996; Smith et al., 1995a).

Fig. 1.

—Generalized Late Cretaceous-early Tertiary lithostratigraphy in northwestern Oman.

Fig. 1.

—Generalized Late Cretaceous-early Tertiary lithostratigraphy in northwestern Oman.

As a consequence of the tectonic history, the stratigraphic record shows a great diversity in its sedimentologic and paleo– geographic patterns and in its fossil content. The regional Wasia/ Aruma unconformity separates uppermost Cretaceous rocks from middle Cretaceous strata (Glennie et al., 1974; Harris et al., 1984; Alsharhan and Nairn, 1986, 1993; Scott, 1990). The magnitude of this unconformity varies from place to place throughout the Arabian Peninsula. In the eastern part of the United Arab Emirates, this unconformity represents the early Turanian to the early Campanian (Alsharhan and Nairn, 1988).

This study focuses on the Maastrichtian carbonates of the Simsima Formation that crop out In the northwestern Oman Mountains (United Arab Emirates territory) (Fig. 2A). Sections from various localities in the Al Faiyah region were sampled and measured (Fig. 2B). Both hand specimens and thin sections were described to determine the lithological variations, microfacies, microfabrics, and carbonate petrography, which could help with diagenetic history and the interpretation of the paleoenvironment of the Simsima Formation at these localities. Such analyses can be used in regional correlation of the studied sections to determine their age and make a regional paleoenvironmental interpretation of the Maastrichtian carbonates in NW Oman Mountains. Our study incorporates previous studies by Skelton et al. (1990), who provided the first detailed sedimentological and facies analysis and published lithostratigraphic sections and faunal lists (especially rudist bivalve faunas). Smith et al. (1995a) studied the fossils of this region and concluded that combining the evidence from ammonites, inoceramid bivalves, echinoids, and global sea-level curves most of the Simsima Formation appears to be in the lower Upper Maastrichtian Anapachydiscus fresvillensis Zone or higher, with which we agree.

Fig. 2.

ALocation map of the study area in northwesternOman Mountains inthe United Arab Emirates BDetailed map of inset area showing locations of different Jebels mentioned inthe text.

Fig. 2.

ALocation map of the study area in northwesternOman Mountains inthe United Arab Emirates BDetailed map of inset area showing locations of different Jebels mentioned inthe text.

Smith et al. (1995b) established the paleoenvironmental setting of Simsima sediments and concluded that the succession was deposited during a single sea-level cycle and that the basal transgression preserves beach hardground faunas and subtidal shoreface sands. Shallow-water offshore sediments deposited below storm wave base make up the majority of the lower beds. The upper beds are composed of semi-reclining rudists and well-sorted calcarenites deposited on stable shoals above active wave base. Although we support most of their paleoenvironmental conclusions, we disagree with them in some minor points, and our views are presented in the lithostratigraphic section below. The taxonomy of rudists by Morris and Skelton (1995), ammonites by Kennedy (1995), nautiloids and inoceramids by Morris (1995a, 1995b) and echinoids by Smith (1995) have been documented in detail in this area and are not repeated in this study.

Lithostratigraphy of the Simsima Formation

The term Simsima was originally introduced by Sugden (in an unpublished report cited in Sugden and Standring, 1975) from the Dukhan-28 well, in western onshore Qatar (lat. 25°24’N; long. 50°45’46“E). These authors designated the Simsima type section in the Dukhan–55 well, between the depths of 353 and 502 m. The first published definition of the Simsima Formation outside this type area was given by Glennie et al. (1974), who erected an outcrop reference section in Oman.

Nolan et al. (1990) questioned the validity of the name Simsima. Although the name was used in unpublished reports by several oil company geologists beginning in 1956, it was first published and validated by Glennie et al. (1974), and a type section was designated. A second section was designated as the type section by Sugden and Standring (1975), who were among the original users of the name. The fact that this section is in a well and that it was not the first well in which Simsima was applied does not invalidate the name. Furthermore, although the Simsima can be correlated with the Aruma Limestone in Saudi Arabia and with the Tayarat Formation in southern Iraq, which were named earlier than the Simsima, Simsima in the U.A.E. is still a valid name that can be used without question. This definition was accepted by Alsharhan and Nairn (1990), who selected a reference sectionof the Simsima Formation in the Shah Field of onshore Abu Dhabi, the only field to produce from this formation to date. This definition of the Simsima is followed here. We are in strong support of the use of the term Simsima because it is well defined in the subsurface (see Sugden and Standring, 1975; Alsharhan and Nairn, 1990; Alsharhan, 1995), has a wide extent, and is the important Upper Cretaceous producing formation in the United Arab Emirates.

The reference outcrop section of the Simsima Formation in the Al Faiyah region in northwestern Oman Mountains used in this study was also used by Nolan et al. (1990). The Simsima can be subdivided into two informal members of distinct character and considerable vertical and horizontal extent. The following is a detailed description of these members.

Lower Member

Type Reference: Jebel Buhays (in A1 Faiyah region), lat. 25°02’N, long. 55°47"E (Fig. 3), 4 km north of A1 Madam village, U.A.E., Dhayed sheet 1:100,000, NG–40–107.

Fig. 3.

—Lithostratigraphy and facies distribution of the Simsima Formation at Jebel Buhays al Faiyah in northwestern Oman Mountains.

Fig. 3.

—Lithostratigraphy and facies distribution of the Simsima Formation at Jebel Buhays al Faiyah in northwestern Oman Mountains.

Thickness: About 34 m.

Geographic Distribu tion: All over the Jebel A1 Faiyah anticlines from Jebel Mullaghah in its northern outcrops to southward through Jebel A1 Faiyah, Thanais, Aqaba, Rumailah, and Buhays (the type locality). Further occurrences are recorded from Jebel Al Rawdah (Oman).

Lithology: Orange yellow, moderately hard algal, orbitoidal limestone, with a rich rudist fauna (Fig. 4A), either fragmented or disarticulated, or in life position as extended banks. It is very rich in macrofossils.

Fossil Content: This member is characterized by the presence of Dictyoptychns morgani and Durania sp. (Fig. 4B, C, D). It also includes other radiolitid rudists, but these are less abundant than the dictyoptychids. This member includes both the Dictyoptychus and the Durania facies of Skelton et al. (1990).

The following fauna is found in the type reference: calcareous worms encrusting the echinoids; bivalves; and gastropods. A very similar fauna is recorded at Jebel Al Rawdah and other areas reported by Ali (1989), Skelton et al. (1990), and Metwalli (1992, 1993).

  1. Rudists: Dictyoptychus morgani (Douvillé), Durania sp., and Hippurites sp.

  2. Other bivalves: Spondylus sp., Mytilus sp., Neithea sp., Agerostrea sp., Pterotrigonia sp., and many others.

  3. Gastropods: Actaeonella sp. (Fig. 4F), Trochus sp., Keilostomata sp., Architectonica sp., Aporrhais sp., Strombus sp., among others (most gastropods are badly preserved internal molds).

  4. Scleractinian corals: Few colonial (Fig. 4E, 4H) and solitary, e.g., Cyclolites sp.

  5. Calcareous sponges.

  6. Foraminifera: Orbitoides media (d’Archiac) (Fig. 4G), Omphalocyclus inacroporus (Lamarck) (Fig. 5A), Lepidorbitoides sp., L. minor (Schlumberger) (Fig. 5B), Siderolites sp. (Fig. 5C), Loftusia minor (Cox).

  7. Echinoids: many regular and irregular echinoids such as Faujasia eccentripora (Lees), Hemipeneustes arabicus (Ali), and Noetlingaster emiratescus (Ali).

  8. Algae: abundant rhodolithic algae and green algae (Fig. 5D).

Fig. 4.

(opposite page)—A) Durania sp. in the Lower Member at Jebel Aqaba section. B) Durania sp. in association with rudists in the Lower Member of the Simsima Formation at Jebel Aqaba Section. C) Bioclastic packstone with large rudistid fragment (width of field about 45 mm). D) Monopleurid rudists within Dictyoptychus sp. in the Lower Member at Jebel Aqaba section. E) Coralline colonies in the Lower Member at Jebel Aqaba. F) Nearly axial section of Actaeonella sp. (x 1.7) at Jebel Aqaba. G) Orbitoides media (D’Archaic) (x 20) at Jebel Aqaba. H) Coralline fragments (x 1.7) at Jebel Buhays.

Fig. 4.

(opposite page)—A) Durania sp. in the Lower Member at Jebel Aqaba section. B) Durania sp. in association with rudists in the Lower Member of the Simsima Formation at Jebel Aqaba Section. C) Bioclastic packstone with large rudistid fragment (width of field about 45 mm). D) Monopleurid rudists within Dictyoptychus sp. in the Lower Member at Jebel Aqaba section. E) Coralline colonies in the Lower Member at Jebel Aqaba. F) Nearly axial section of Actaeonella sp. (x 1.7) at Jebel Aqaba. G) Orbitoides media (D’Archaic) (x 20) at Jebel Aqaba. H) Coralline fragments (x 1.7) at Jebel Buhays.

Fig. 5.

(opposite page)—A) Omphalocyclus macroporus (Lamarck) (x 20). B) Lepidorbitoides sp. cf. L. minor (Schlumberger) and recrystallized cavities of Acteonella sp. at the Lower Member of the Simsima Formation (x 20) at Jebel Buhays. C) Siderolites sp. (x 20) and Clymphalocylus macroporus (Lamarck). D) Dasycladacean algae in bioclastic peloidal packstone at Jebel Buhays (x 20). E) The lowermost part of the Lower Member, with Actaeonella sp. with its longitudinal axis parallel to this bedding plane in algal (rhodolith)–rich limestone at Jebel Buhays. F) The Lower Member (1) and the basal part of the Upper Member (2), Simsima Formation at Jebel Buhays. Note the presence of two Dictyoptychus in the Lower Member. G) The contact between the Simsima (S) and Qahlah (Q) Formations at Jebel Buhays. Note the sharp linear contact and Acteonella in algal (rhodolith) limestone in the lower part of the Lower Member.

Fig. 5.

(opposite page)—A) Omphalocyclus macroporus (Lamarck) (x 20). B) Lepidorbitoides sp. cf. L. minor (Schlumberger) and recrystallized cavities of Acteonella sp. at the Lower Member of the Simsima Formation (x 20) at Jebel Buhays. C) Siderolites sp. (x 20) and Clymphalocylus macroporus (Lamarck). D) Dasycladacean algae in bioclastic peloidal packstone at Jebel Buhays (x 20). E) The lowermost part of the Lower Member, with Actaeonella sp. with its longitudinal axis parallel to this bedding plane in algal (rhodolith)–rich limestone at Jebel Buhays. F) The Lower Member (1) and the basal part of the Upper Member (2), Simsima Formation at Jebel Buhays. Note the presence of two Dictyoptychus in the Lower Member. G) The contact between the Simsima (S) and Qahlah (Q) Formations at Jebel Buhays. Note the sharp linear contact and Acteonella in algal (rhodolith) limestone in the lower part of the Lower Member.

Boundaries: The Lower Member lies unconformably on the Qahlah Formation, and its boundary is sharp and planar (Fig. 5 F,G). The upper boundary in contrast is gradational and is based on the presence of a distinct local erosional surface and decrease inDictyoptychus.

Sedimentary Environment: The Lower Member was deposited in a shallow, open marine setting with normal marine salinities and moderate to high energy. This conclusion is based on the following criteria.

  1. The dominant dictyoptychids and occasional radiolitid rudists (Durania) (Fig. 4B) grew in abundance in a relatively shallow marine environment. The radiolitid rudists were more common at shelf margins and in foreslopes, where they were associated with and were gradually replaced by corals, spongiomorphic hydrozoans, red algae, and sponges; allbiota inhabited water of normal marine salinity. Comparable conditions are thought to have existed in the study area, where scleractinian colonial mounds have replaced the radiolitid rudists (in the Jebel Al Faiyah section).

    The Lower member is comparable with facies of shelf-margin deposits. The position within this wide facies zone is controlled by the paleorelief, which varied irregularly and changed rapidly from one place to another. In areas of deeper water, Dictyoptychus morgani, Plagioptychus sp., and radiolitids accumulated. In shallower areas, bivalves and gastropods were the dominant fauna associated with large foraminifera (orbitoids) and red algae.

  2. The lower member is composed of bioclastic packstones containing worn grains and rare to very rare planktonic foraminifera that confirm open-marine depositional conditions.

Upper Member

Reference Section:

It is chosen at Jebel Al Faiyah, lat. 25°04’N, long. 55°49’E (Fig. 6), 8 km N of A1 Madam village, U.A.E., Dhayed sheet 1:100,000, NG–40–107.

Fig. 6.

—Lithostratigraphy and facies distribution of the Simsima Formation, northwestern Oman Mountains.

Fig. 6.

—Lithostratigraphy and facies distribution of the Simsima Formation, northwestern Oman Mountains.

Thickness:

About 60 m.

Geographic Distribution:

It is well exposed in the Jebel A1 Faiyah area, at Jebel A1 Rawdah, and southward of the A1 Ain– Buraymi area (Fig. 2B).

Lithology:

It is composed of thick, well–bedded, highly fossil– iferous limestone. The limestone is nodular, bioturbated at the base, cross-bedded in the middle, and bioturbated, thin-bedded and dolomitic in its upper parts. Orbitoides are the most abundant microfossils. The macrofossils decrease rapidly upward.

Fossil Content:

Except for the absence of rudistid banks, few scattered, disarticulated Durania sp. are recorded within the lowermost 5 m of the member. Macrofossils dominate the lower part of the member and include clusters of Spondylus sp. The well cemented character of the member prevents collection of well preserved macrofossils. However, these latter are present in some diversity, especially in the lower part of the member.

Boundaries:

The lower boundary of the Upper Member is based on gradational contact with the Lower Member, while in this region the upper boundary can be easily separated from the Muthaymimah carbonate above by an angular unconformity.

Sedimentary Environment:

The Upper Member with its gradational contact with the underlying Lower Member represents a gradual transgressive phase of the Simsima Sea in the study area. This phase reached its maximum extent in A1 Rawdah area, where the orbitoidal packstones change gradually upward to planktonic foraminiferal packstones. The upper member is interpreted to have accumulated in an open-shallow-shelf environment, below wave base with moderate currents and normal salinity, and in the photic zone without (or with very rare) terrigenous influx. This conclusion is based on the following criteria.

  1. The orbitoidal packs tone (partly grainstone) is the most dominant type. It is associated with the following types: bioclastic packstone, algal bioclastic packstone, and peloidal bioclastic packstone. These microfacies types consist mainly of bioclasts (no micrite), and most Orbitoides sp. (or any other elongated skeletal grains) are oriented or even imbricated.

  2. The gradual upward decrease of algae and the gradual increase of planktonic foraminifera.

  3. The absence of interbedded terrigenous influx.

  4. The presence of lamination and cross-bedding in the lower half of the member.

Sedimentary Facies Analysis

Eighty–five thin sections from the Simsima Formation were used for facies analysis. They are described systematically using the check list proposed by Flugel (1982) (see Figs. 3 and 6). The system of nomenclature and classification is after Dunham (1962) and Embry and Klovan (1971).

The lithology is consistent throughout the various areas sampled. Grainstone and packstone dominates, and wackestones are rare. Bioclastic grains predominate except in a few samples in which oolites and peloids are subordinate. The peloids may be highly micritized skeletal fragments (e.g., echinoid spines), but the lack of internal fabric leaves open the possibility of a nonskeletal origin. The matrix consists most commonly of fine to medium calcite spar; micrite mud makes up the matrix of some packstones.

The Simsima Formation contains a wide range of fauna. Orbitoides sp. and rudist and scleractinian corals are the most abundant skeletal grains. Echinoderm platelets and spines, dasycladacean green algae, and a number of foraminifera, including miliolids and textularids are all common. Most facies are poorly sorted; larger grains, such as coral and brachiopod fragments, Discocyclina sp. tests, large echinoderm platelets, bryozoans, and red algae were commonly mixed with the small planktonic foraminifera and bioclasts.

Microfabric Types

Skeletal Grains:

These grains show the greatest variation and high diversity; both biomorphs and bioclasts are well represented. The skeletal grain assemblages are mainly polymictic, but monomictic groups also occur, including Orbitoides and rhodolites. The rhodoliths are the most abundant algal group; the dasycladacean algae are either secondary or, in a few cases, are primary rock constituents. The skeletal grains are dominated by the benthonic sessile groups; the vagile groups; such as regular echinoids and gastropods, are less common. Planktonic foraminifera are present in negligible percentages.

The roles of the skeletal grains can be arranged as follows, in order of decreasing importance: essential rock-building constituents (most systematic groups), sediment binders (algal boundstone), sediment bafflers (milleporid and/or coralline bafflestone), and framework builders (colonial corals framestone). In a few cases biogenic encrustation of bioclasts of bivalves is present. The main encrusting taxa are membraniporiform bryo– zoans, red algae, and encrusting forams. Most skeletal grains are enveloped by thin micritic rims. Aragonitic skeletal grains, such as molluscs, scleractinian corals, and millepores, are usually recrystallized. The original microstructure of the oysters is normally preserved, but partial recrystallization is common. The polymictic nature of the skeletal grains are in most cases the reason for their poorly sorted state (from annelids to rudist debris). Rounded bioclasts are very rare. The following is the systematic grouping of the skeletal grains, arranged according to their frequencies: Orbitoides sp., rhodoliths, rudists and other bivalves, gastropods, miliolids, rotalids, textularids, dasycladacean algae, corals, bryozoans, ostracodes, and annelids.

Peloids:

These occur generally in small quantities. Both algal peloids and fecal pellets are present. They are mostly sorted, oval or subrounded, scattered, and composed mainly of homogeneous micrite. The peloids are usually associated with rhodoliths.

Aggregate Grains:

These are entirely lumps, agglutinated particles mostly of bioclasts and rarely extraclasts. The agglutinated particles are either truncated or not, are enclosed by a sparitic cement, and are dominant only in the orbitoidal bioclastic microfacies. Where they are present, the aggregate grains constitute less than 10% of the total particles.

Oncoids:

These occur as a tiny percentage in a very few of the samples. They are represented by algal oncoids (rhodoliths) embedded in sparite. They have a deformed, irregular shape. They are up to 10 mm in size where they have a regular structure. The laminae of the pisolites are crinkled. The nuclei correspond to particles of the associated materials.

Cortoids:

Many bioclasts have micrite envelopes. In some cases the whole particle is micritized. These ovate grains, or cortoids, are always found in association with rhodoliths, with highly diverse faunal assemblage. They show no indication of transportation. Locally echinoid fragments have micritic envelopes.

Lithoclasts:

Both intraclasts and extraclasts are present, and where this mixture occurs the former are dominant. In a few cases the clasts are truncated along their edges. The components that form the lithoclasts are of the same material as the associated particles and have the same color. Few features have been recognized. Both the lithoclasts and the other particles have similar cement. They are characterized by an irregular nodular, rounded shape, reminiscent of grapestones. No parallel orientation or cross-bedding can be recognized. These grains are common throughout the studied sections.

Terrigenous Particles:

These particles are of ophiolitic origin and are concentrated in the lower parts of the Simsima, especially near the contacts with the underlying Qahlah Formation or Semail Ophiolite. These particles are mainly composed of antigorite and are ill-sorted, disoriented, subrounded to subangular, and disseminated. A few small, sorted, subangular, disoriented, and disseminated chromite particles have been recognized throughout the studied section and are common especially in the upper parts.

Fabric:

The particles in almost all studied sections are grain-supported. The contact types between the particles include point and tangential. Orientation, imbrication, and geopetal structures are common in the studied sections.

Carbonate Cement:

Cements include a blocky cement, of which several generations can be recognized. A micritic cement is common, and cements in optical continuity with the echinoid fragments are rare. The cement between the particles does not correspond to the cement within the particles. There is a sharp contact between the cements and the particles. The crystal size of the cements increases away from the walls of the original cavities being filled.

Microfacies Types

In general the Simsima Formation in the studied sections is dominated by three groups of carbonate: packstone, boundstone, and dolostone. Each group can be subdivided on the basis of the frequency of the major carbonate particle components, except for the dolostone group.

Packstone Group

This group occurs in about 90% of the studied samples and comprises predominantly Orbitoides sp., miliolids, planktonic foraminifera, coralline algae, and echinoderms. Also present are bryozoans, brachiopods, and nonskeletal components, such as fecal pellets and peloids.

  1. Orbitoidal Packstones Microfacies: The orbitoidal packstone microfacies is the dominant microfacies in the Simsima Formation and occurs in Jebels Buhays, Thanais, Aqaba, A1 Faiyah, and A1 Rawdah. It constitutes 45% of the rocks studied in thin section. The most abundant skeletal grains and/or carbonate grains are used to divide the packstones into the following microfacies subtypes: bioclastic orbitoidal packstones (Fig. 7A), bioclastic algal orbitoidal packstones (Fig. 7B), orbitoidal packstones (Fig. 7C), algal orbitoidal packstones (Fig. 7D), and peloidal bioclastic orbitoidal packstones (Fig. 7E).

  2. Bioclastic Packstones Microfacies: The bioclastic packstone microfacies is the second most abundant microfacies type, constituting 38% of the thin sections studied in Jebels Buhays, Thanais, Aqaba, A1 Faiyah, and A1 Rawdah. The following microfacies subtypes can be distinguished: algal bioclastic packstones (Fig. 7F), peloidal bioclastic packstones (Fig. 7G), algal orbitoidal bioclastic packstones (Fig. 7H), and algal peloidal bioclastic packstones (Fig.7I).

  3. Algal Packstones Microfacies: Packstone with abundant algal remains constitutes 4% of the studied thin sections and occurs in Jebels Buhays and Aqaba only. It includes the following two subtypes: orbitoidal algal packstones (Fig. 8A) and bioclastic algal packstones (Fig. 8B).

    Fig. 8.

    (opposite page)—A) Orbitoidal algal packstone (8A to 81 x 20). B) Bioclastic algal packstone. C) Planktonic foraminiferal bioclastic packstone with echinoid and pelecypod. D) Pseudoguembelina palpebral (Bronnimann and Brown); note the micritic rim envelopes the specimen. E, F) Rugoglobogerina sp. cf. R. hexacamerata (Bronnimann); note the micritic filling of the planktonic foraminifera and the surrounding sparitic cement. G) Globolruncana sp. cf. G. aegyptiaca (Nakkady); note the micritized planktonic foraminifera and the loosely packed skeletal grains supported in sparitic cement. H) Globotruncanita sp. cf. G. elevata (Brotzen). I) Fine– to medium-crystalline rhombic dolomite crystals.

    Fig. 8.

    (opposite page)—A) Orbitoidal algal packstone (8A to 81 x 20). B) Bioclastic algal packstone. C) Planktonic foraminiferal bioclastic packstone with echinoid and pelecypod. D) Pseudoguembelina palpebral (Bronnimann and Brown); note the micritic rim envelopes the specimen. E, F) Rugoglobogerina sp. cf. R. hexacamerata (Bronnimann); note the micritic filling of the planktonic foraminifera and the surrounding sparitic cement. G) Globolruncana sp. cf. G. aegyptiaca (Nakkady); note the micritized planktonic foraminifera and the loosely packed skeletal grains supported in sparitic cement. H) Globotruncanita sp. cf. G. elevata (Brotzen). I) Fine– to medium-crystalline rhombic dolomite crystals.

  4. Microporous Planktonic Foraminiferal Bioelastie Packstone: Four percent of the studied thin sections canbe classified as plank tonic foraminiferal bioclastic packstone microfacies Fig 8C-H This faciestype is recorded only from the uppermost part of the Simsima Formationat the Jebel Al Rawdah Orbitoides biomorpha foraminiferal bioclasts and echinoid bioclasts are associated with it.

Fig. 7.

—A) An axial section of Orbitoides media (D’Archaic) (7A to 71 x 20). B) Bioclastic algal orbitoidal packstone with rhodolith fragments. C) Axial sections in Lepidorbitoides sp.; notice the orientation and partial imbrication of the orbitoidal packstone. D) Lepidorbitoides sp. with rhodolith fragments of algae. E) Peloidal bioclastic orbitoidal packstone. F) Algal bioclastic packstone and Omphalocycus macroporous (Lamarck). G) Peloidal bioclastic packstone. H) Algal orbitoidal bioclastic packstone. I) Algal peloidal bioclastic packstone

Fig. 7.

—A) An axial section of Orbitoides media (D’Archaic) (7A to 71 x 20). B) Bioclastic algal orbitoidal packstone with rhodolith fragments. C) Axial sections in Lepidorbitoides sp.; notice the orientation and partial imbrication of the orbitoidal packstone. D) Lepidorbitoides sp. with rhodolith fragments of algae. E) Peloidal bioclastic orbitoidal packstone. F) Algal bioclastic packstone and Omphalocycus macroporous (Lamarck). G) Peloidal bioclastic packstone. H) Algal orbitoidal bioclastic packstone. I) Algal peloidal bioclastic packstone

Boundstone Group

The boundstone type constitutes 7% of the studied thin sections and is recorded inJebels Buhays Aqaba and Al Faiyah It is of limited occurrence and can be considered as a lateral and local replacement of the packstone group Scleractinian corals oncolites rhodoliths and rudists are the main constituents of these boundstones Milleporids are also present but are subordinate The following microfacies types can be distinguished.

  1. Milleporid: Milleporid hydrozoans are one of the important constituents of recent and old reefs Few large milleporid bioclasts are recorded from the basal part of the Lower Member Its reticulate structure as well as the presence of large and small pores gastopores and dachtylopores respectively confirms its identification.

  2. Coralline: Fragments of scleractinian corals are recognized in the fieldas in Jebel Aqaba and inthinsections as in the lower part of Jebel Buhays section Dendroid scleractinian corals also occur and actas sediments bafflers.

  3. Algae: The frequent occurrence of rhodoliths in the basal part of the Lower Member presents a good example of the algal boundstone microfacies type.

Dolostone Dolosparite Group

The dolostones have a very limited occurrence (1%) and are very patchy intheir distribution Some samples were completely dolomitized by subhedral to euhedral zoned rhombic dolomite Fig 8I There are no clues as to their precursor Moreover few packstones are partially dolomitized Dedolomitization has oc curredand neomorphic calcite crystals may cutacross the dolomite crystal boundaries Within the dolomite crystals limonite may parallel growth lines or in some places limonite infills secondary porosity associated with the dolomite.

Diagenetic History

The diagenetic sequence involves initial marine deposition followed by marine cementation, evidenced by fibrous cements encrusting echinoid spines and the interiors of foraminiferal chambers. Micritization, which has obliterated much of the original fabric of many skeletal grains, and the formation of micrite envelopes also occurred at this time (Friedman, 1964; Bathurst, 1964, 1966; Friedman et al., 1971). Recrystallization of marine cements to fine- to medium-grained calcite spar is attributed to meteoric diagenesis (James and Choquette, 1990), which has occluded some of the original porosity. However, subsequent leaching of bioclasts and matrix spar has created good mold ic and intraparticle porosity, which is now commonly cement-filled. Coarse, equant calcite spar precipitated within moldic and inter- particle pores during early burial diagenesis.

Reexposure of the rocks at the surface has resulted in a suite of late diagenetic changes. Partial dissolution of the void-filling coarse calcite spar occurred during a second phase of leaching. The partially leached voids do not contain hydrocarbons and are thus inferred either to postdate hydrocarbon emplacement or to have been outside of migration paths.

Precipitation of coarse calcite spar was followed by local formation of rhombic, zoned dolomite within the limestone matrix and is embedded within coarse calcite spar. The rhombs occurred scattered throughout the matrix of muddy packstones and clustered within the matrix of sparry grainstones. Those embedded within the coarse calcite spar void-fill occurred as isolated single crystals. Rare unzoned rhombic dolomite was also observed in association with zoned rhombic dolomite.

Iron staining and hematite formation postdate emplacement of hydrocarbons and are probably related to a period at or near the groundwater table. Rare dedolomitization also occurred during a second phase of meteoric diagenesis, because few of the zoned rhombs were shown to be calcific through staining of petrographic thin sections with Alizarin Red-S.

Finally, local chertification is in the form of megaquartz and chalcedonic laths in many samples, and in rare cases quartz silt fills matrix vugs. Chertification occurred mainly in compaction and fractures and preferentially within the intraparticle pores of echinoid spines. Evidence for the late diagenetic timing for chert was found where chert fillsopen seams and voids created by leaching of coarse calcite spar.

Moldic and interparticle pores are the most important porosity types in the Simsima. Subordinate intraparticle porosity also occurs where chambers of principally foraminiferal and bryozoan tests are not filled by calcite spar. Cementation by coarse calcite spar during early burial diagenesis reduced porosity in some samples. Leaching of coarse calcite spar from pores and from many fractures, and pressure solution seams have created additional porosity, although this occurred after hydrocarbon migration and entrapment.

Conclusions

  1. The Maastrichtian Simsima Formation, from the northwestern Oman Mountains of the United Arab Emirates, consists of bioclastic packtones and grainstones with minor wackestones exhibiting great species diversity and representing shallow, open marine depositional conditions. The limestones are rich in foraminifera (e.g., Orbitoides sp. and miliolids) as well as rudists, scleractinia, and echinoderm platelets and spines. A range of other planktonic foraminifera, brachiopods, bryozoans, red algae, dasycladacian green algae, and bivalve fragments also occur.

  2. Initial deposition in a shoal–water, open marine setting was followed by marine cementation and micritization. Recrystallization of marine cements led to fine– to medium-grained calcite spar, which occluded much of the original porosity. However, leaching of bioclasts and some of the sparry matrix created good moldic and interparticle porosity. Cementation by coarse, equant calcite spar within moldic and interparticle was also found.

  3. Reexposure of the rocks at the surface has resulted in numerous late diagenetic changes: a) a second phase of leaching that has removed a significant proportion of the void-filling spar; b) formation of zoned dolomite has occurred locally, with some rhombs embedded in the void-filling calcite spar; c) iron staining and hematite formation; d) rare dedolomitization was observed where zoned rhombs are now calcific; e) local chertification has occurred in the form of megaquartz and chalcedonic laths; and f) rare quartz silt fills matrix vugs.

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Acknowledgments

We express our great appreciation to C. G. St. C. Kendall and M. A. Boukhary for their invaluable help in reading the manuscript and their many useful suggestions. Andrew Smith, Peter Skelton, and Robert Scott provided an excellect critical review and valuable comments which improved this paper. We thank Bruce A. Masters for identifying some of the Simsima forams.

Figures & Tables

Contents

GeoRef

References

References

Ali
,
M.S.M.
,
1989
,
Late Cretaceous echinoids from Gebel El Rawdah
,
Hatta area
,
U.A.E.
:
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412
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Alsharhan
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A.S.
,
1995
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Alsharhan
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Nairn
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A.E.M.
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S.J.Y.
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242
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89
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Glennie
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K.W.
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M.
Pilaar
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,
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S.
Seiglil
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Schneidermann
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N.
,
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,
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Choquette
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P.W.
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,
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I.A.
Morrow
,
D.W.
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, p.
75
112
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Kennedy
,
W.J.
,
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,
Maastrichtian ammonites from the United Arab Emirates-Oman Border region
:
Natural History Museum [London], Bulletin
 ,
v. 51
, no.
2
, p.
241
250
.
Metwally
,
M.H.
,
1992
,
Late Cretaceous (Maastrichtian) bivalves from the northwestern flank of the Oman Mountains, United Arab Emirates
:
Neues Jahrbuch fur Geologie und Palaontologie, Abhandlungen
 ,
v. 184
, p.
123
139
.
Mewally
,
M.H.
,
1993
,
Cretaceous gastropods from the northwestern flank of the Oman Mountains, United Arab Emirates
:
Zagazig University, Bulletin of the Faculty of Science
 ,
v. 15
, p.
333
359
.
Morris
,
N.J.
,
1995a
,
Maastrichtian nautiloids from the United Arab Emirates-Oman border region
:
Natural History Museum [London], Bulletin
 ,
v. 51
, no.
2
, p.
251
256
.
Morris
,
N.J.
,
1995b
,
Maastrichtian Inoceramidae from the United Arab Emirates-Oman border region
:
Natural History Museum [London], Bulletin
 ,
v. 51
, no.
2
, p.
257
266
.
Morris
,
N.J.
Skelton
,
P.W.
,
1995
,
Late Campanian-Maastrichtian rudists from the United Arab Emirates-Oman border region
:
Natural History Museum [London], Bulletin
 ,
v. 51
, no.
2
, p.
277
305
.
Murris
,
N.J.
,
1980
,
Middle East
:
Stratigraphic evolution and oil habitat
:
American Association of Petroleum Geologists, Bulletin
 ,
v. 64
, p.
597
618
.
Nolan
,
S.C.
Skelton
,
P.W.
Clissold
,
B.P.
Smewing
,
J.D.
,
1990
,
Maastrichtian to early Tertiary stratigraphy and Palaeogeography of the Central and northern Oman Mountains
, in
Robertson
,
A.H.F.
Searle
,
M.P.
Ries
,
A.S.
, eds.,
The Geology and Tectonics of the Oman Region
 :
Geological Society of London
, Special Publication
49
, p.
495
519
.
Noweir
,
M.A.
Aisharhan
,
A.S.
Boukhary
,
M.A.
,
1998
,
Structural setting and stratigraphic evolution of the northwestern Oman Mountain Front, United Arab Emirates
:
GeoArabia
 ,
v. 3
, p.
387
398
.
Scott
,
R.W.
,
1990
,
Chronostratigraphy of the Cretaceous carbonate shelf, southeastern Arabia
, in
Robertson
,
A.H.F.
Searle
,
M.P.
Ries
,
A.C.
, eds.,
The Geology and Tectonics of the Oman Region
 :
Geological Society of London, Special Publication
49
, p.
89
108
.
Skelton
,
P.W.
Nolan
,
S.C.
Scott
,
R.W.
,
1990
,
The Maastrichtian transgression onto the northwestern flank of the Proto-Oman Mountains: Sequences of rudist-bearing beach to open shelf facies
, in
Robertson
,
A.H.F.
Searle
,
M.P.
Ries
,
A.S.
, eds.,
The Geology and Tectonics of the Oman Region
 :
Geological Society of London, Special Publication
49
, p.
521
547
.
Smith
,
A.B.
,
1995
,
Late Campanian-Maastrichtian echinoids from the United Arab Emirates-Oman border region
:
Natural History Museum [London], Bulletin
 ,
v. 51
, no.
2
, p.
121
240
.
Smith
,
A.B.
Morris
,
N.J.
Kennedy
,
W.J.
Gale
,
A.S.
,
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