M. M. Imam

Stratigraphy and Microfacies types of the Oligocene sequence at Gabal Bu Husah, Marada Oasis, south Sirte Basin, Libya.

M. M. Imam & M. A. Galmed

KEY WORDS : BENTHONIC FORAMINIFERA – MICROFACIES – MARADA OASIS - LIBYA

Summary: Stratigraphic and microfacies investigations carried out on the Oligocene sequence exposed at Gabal Bu Husah, northwest of Marada Oasis, south of Sirt Basin, Libya, showed that the Oligocene sequence conformably overlies the Late Eocene Thamat Formation and unconformably underlies the Early to Middle Miocene Marada Formation (Qarat Jahannam Member). The lithostratigraphic studies carried out on the Oligocene sequence revealed the presence of two main rock units, from base to top: 1) Umm Ad Dahiy Formation (Early Oligocene, Rupelian) and 2) Bu Hashish Formation (Late Oligocene, Chattian).

The Oligocene sequence yields a rich benthonic and larger foraminiferal assemblages that permitted to identify fifty-one benthonic and larger foraminiferal species. The foraminiferal analysis allowed to subdivide the sequence into three main local benthonic and larger foraminiferal assemblage zone, arranged stratigraphically from base to top: 1) Elphidium minutum zone comprises the Umm Ad Dahiy Formation. 2) Miogypsinoides complanatus / Nonion granosus assemblage zone covers the lower two thirds of the Bu Hashish Formation. 3) Zone with abundant Nummulites spp., includes the upper part of the Bu Hashish Formation. The paleoenvironmental significance of the recorded species is also described and discussed.

The microfacies studies led to the recognition of eleven microfacies types. These microfacies types indicate that the lower part of the Umm Ad Dahiy Formation was deposited in a shallow warm marine environment but the conditions changed to fluviomarine in the uppermost part. While the Bu Hashish Formation was deposited in shallow marine, inner shelf environments (as indicated by the operculinid limestone) with probable hiatus in its middle part as evident by the presence of Nummulitic coquina and gypsum beds indicates a lagoonal environments. After returning to a shallow marine environment at the end of Oligocene time where the marine sedimentation ceased.

INTRODUCTION

The NW – SE striking Sirte basin, is a complex block faulted and down warped structure. It is originated at the end of Mesozoic in the northern part of the African platform, between the Sahara shield and the Tethyan sea. The filling of the basin included sediments ranging in age from Late Cretaceous to Pleistocene. Therefore, the Late Cretaceous and Early Tertiary rocks are hydrocarbon bearing (BARR & WEEGAR, 1972, BENFIELD & WRIGHT, 1980). Gabal Bu Husah, which represents the study area, lies in the southern portion of Sirte basin and is located in the north west of Marada Oasis (Fig.1).

Introduction:
Gabal Bu Husah is 6 km wide and 10 km long (Lat. 30 04 N and Long.17 12 E) and composed essentially of marine Oligocene deposits, predominantly of carbonate sediments (limestone, dolomite) and subordinate argillaceous limestone and clastic rocks.

Only regional description were given by DESIO (1935), MAGNIER & DUVAL (1958), BUROLLET (1960), GOUDARZI (1970), SRIVASTAVA (1980) and HLADIL et al. (1991). So this work is considered as the first comprehensive work on the Oligocene sequence in that area. In addition to no detailed foraminiferal studies have also been carried out. Two stratigraphic surface sections, Gabal Bu Husah and Gabal Bu Hashish, were carefully sampled andinvestigated, from these sections, one composite section was established representing the marine Oligocene sequence. The present work aims to focus on the different lithostratigraphic units of the marine Oligocene sequence there and also to determine the different benthonic foraminiferal and planktonic content, as well as to establish local biozone. The main depositional environments of this sequence are deduced by the study and description of the main microfacies types encountered in the different Oligocene rock units.


LITHOSTRATIGRAPHY

The lithostratigraphic investigations carried out on the marine Oligocene sequence exposed at Gabal Bu Husah, northwest Marada Oasis, showed that the Oligocene deposits conformably overly the Eocene Thamat Formation (limestone to argillaceous limestone and subordinate shales) and unconformably underlay the Early to Middle Miocene Marada Formation (Qarat Jehannam Member) (Table, 1; Fig. 2). Field investigations and lithostratigraphic studies on the Oligocene sequence led to the recognition of two main rock units, arranged from base to top as follows: 1) Umm Ad Dahiy Formation (Early Oligocene, Rupelian) and composed mainly of dolomitic limestone and subordinate sandstone and silty claystone. 2) Bu Hashish Formation (Late Oligocene, Chattian), represented commonly by limestone and dolomitic limestone with nummulitic coquina and subordinate argillaceous sandstone and gypsum.

1- Umm Ad Dahiy Formation (Early Oligocene, Rupelian):

This rock unit was originally defined as Dor El Abd Formation by MAGNIER & DUVAL (1958) for the Upper Eocene to Lower Oligocene strata exposed at its type locality Dur Umm Ad Dahiy in the southern part of the Nuwfaliyah area (Lat. 30 03 15 N and Long. 17 15 45 E). BUROLLET (1960) and GOUDARZI (1970) later adopted this formation name. SRIVASTAVA (1980) reinvestigated this formation in its type section and changed the name into the present name "Umm Ad Dahiy” Formation and assigned it to an Early Oligocene age. In the study area, the Umm Ad Dahiy Formation attains a thickness of 40m and is composed mainly of yellowish white fossiliferous limestone, rich in benthonic foraminifera and yellowish to greyish white, massive sandy dolomitic limestone, highly fossiliferous with hermatypic corals (Fig. 3), and Nummulites spp., with yellow, massive, sandy limestone and sandy claystone in the lower part of the formation. This sequence is commonly interbedded with subordinate greyish white, massive, argillaceous limestone highly fossiliferous with planktonic foraminifera (Pl.1, Fig.1). However, toward the upper part of this rock unit, the lithofacies is represented by yellow, calcareous, cross-bedded, pebbly, fine grained sandstone and thin bed of greenish grey, fissile, fossiliferous shale overlain by 1m thick nummulitic coquina bed in the uppermost part of the formation.

This thin nummulitic coquina bed is considered as a marker horizon in the field separating the Umm Ad Dahiy Formation from the overlying Bu Hashish Formation (Pl. 1 ,Fig.1) denoting a littoral or paralic environment. The Umm Ad Dahiy Formation contains rare megafossils e.g. Echinolampus libycus LORIOL, Echinolamps beaumonti AGASSIZ, “Lopha” martinsi D'ARCHIAC, Lucina sp., Balanus sp.. Some scleactinian corals have been also recorded from the sandy dolomitic limestone beds in the lower two thirds of Umm Ad Dahiy Formation: Stylophora thirsiformis (MICHELLOTTI), Stylophora parvist (CHEVALIER), Astraepora decaphylla (REUSS), Defranica irregularis, Theigioastraea variabilis (SISMONDIA), Heliastraea mellachica (GREGORY), Meandrina sp, and Porites sp (Fig. 3). These types of corals could be considered as small reef patch that developed in well-circulated, relatively shallow marine environments. The overall depositional environments of the Umm Ad Dahiy Formation was shallow marine at its base, followed by transgression with deep marine deposits in the form of argillaceous limestones. The regression terminated with paralic to fluviomarine environments. The Umm Ad Dahiy Formation is of Early Oligocene (Rupelian) age.

2- Bu Hashish Formation (Late Oligocene, Chattian):

This rock unit was originally described and introduced as Bu Hashish Formation by MAGNIER & DUVAL (1958) at its type locality Dur Bu Hashish (Lat. 30 10 16" N and Long. 17 14 21" E). They assigned it a Late Oligocene in age. BUROLLET (1960) and GOUDARZI (1970) later adopted this formation name. SRIVASTAVA (1980) modified the original definition and renamed as "Bu Hashish" Formation, and supported that by another measured section at Al Nuwfaliyah area ( Lat. 30 11 01 N and Long. 17 14 15 E).

The 30 m thick Bu Hashish Formation is represented in its lower two thirds by yellowish white, hard, massive, partly fossiliferous, cross-bedded, calcareous sandstone at the base followed upwards by yellowish white, sandy dolomitic limestone and hard, fossiliferous operculinid limestone beds with subordinate greyish white, massive, fossiliferous, argillaceous limestone interbeds. This sequence is overlain by nummulitic coquina bed and terminates upwards with gypsum and friable, yellow, nonofossiliferous, calcareous sandstone bed with thin beds of sandy siltstone (Fig. 3). An oyster bank and sandy limestone beds of the Early to Middle Miocene Marada Formation (Qarat Jehanam Member) unconformably overlie this succession. The operculinid limestone beds are rich in megafossils: Lucina rai (OPPENHEIM), Macrosolen uniradiatus Blanckenhorn, Turritella cf. pharaonica COSSEM, Echinolampus libycus LORIOL, Gisortia depressa (SOWERBY) and Tellina sp..

The Bu Hashish Formation which considered as carbonate dominated sequence is paleoecologically interpreted to have been deposited in environments ranging from shallow marine in its lower part to paralic in the middle part as indicated by the nummulitic coquina and overlain by hypersaline lagoonal environments then returning to fluviomarine at the end of Oligocene where the marine conditions ceased. This sequence is influenced by slightly deep marine conditions shown by the presence of argillaceous limestone beds (transgressive phase). The age of Bu Hashish Formation is considered as Late Oligocene (Chattian) based on the opinions of different authors and supported in the present work by its stratigraphically important large benthonic foraminifera content such as Nummulites fichteli MICHELOTI and Miogypsinoides complanatus SCHLUMBERGER, as well as the presence of some diagnostic planktonic foraminiferal content of definitely Late Oligocene (Chattian).


MICROFAUNAL ASSEMBLAGE AND BIOSTRATIGRAPHY

Sixty samples have been collected from the marine Oligocene sequence, the sample collection is mainly based on the lithologic variation (40 samples from Gabal Bu Husah and 20 samples from Gabal Bu Hashish). Only 30 samples from Gabal Bu Husah were prepared for their microfaunal content. Fifty-one benthonic foraminiferal species have been recognized and identified (15.5% arenaceous forams and 84.5% calcareous species, the number of calcareous forams is higher than these of arenaceous ones). The most important index taxa are presented in Pl. 1. The vertical distribution of the recorded species is depicted in Figures 4 & 5. Based on the smaller and larger benthonic foraminiferal content of the studied samples, the marine Oligocene sequence exposed in Gabal Bu Husah, is subdivided into three main local biozones, arranged stratigraphically and described from older to younger as follows:

1) Elphidium minutum Zone:
This interval zone completely covers the middle part of Umm Ad Dahiy Formation, including the stratigraphic interval from sample no. 33 to 42 (20 m thick) and is defined by the total range of the nominate zonal taxon. This biozone is rich in benthonic foraminifers, diagnostic ones are: Triloculina sp., Valvulina flexilis CUSHMAN & RENZ, Cyclimina sp, Reussella spinulosa (REUSS), Discorbis pervoalis (TERQUEM), Discorbis obtusus CUSHMAN, Rotalia trochus ROEMER, Elphidium minutum (REUSS), Elphidium macellum (FICHTEL & MOLL), Cibicides micrus BERMUDEZ, Bolivina advena CUSHMAN, Lenticulina sp, Dorothia chapapotensis (COLE), Dorothia filiformis, Haplophragmoides sp, Gyrodina multicamerata KLEINPELL, Rotalia lithothamnica UHLIG, Siphonina reticulata CZIZEK, Nonion boueanum D'ORBIGNY, Ammonia beccarii LINNÉ, Stilostomella verneuilli D'ORBIGNY and Nodosaria capitata BOLLI .

The interbedded argillaceous limestone beds in the Umm Ad Dahiy Formation yielded a low diverse and poor planktonic foraminiferal assemblage, excellent age indication e.g.: Cassigerinella chipolensis CUSHMAN & PONTON, Globorotalia opima nana BOLLI, Globigerina yeguaensis BOLLI, Globigerina tripartita BOLLI, Pseudohastigerina micra COLE, Globorotalia increbescens (BANDY), Globigerina parva BOLLI and Globigerina euapertura JENKINS. This planktonic foraminiferal association, inspite of its low diversity and small number of individuals, indicates an Early Oligocene age (Rupelian) and could be compared with the Cassigerinella chipolensis / Pseudohastigerina micra Zone of BOLLI & SAUNDERS (1985) and equivalent to the standard planktonic foraminiferal zones P18/19 of BLOW (1969,1979).

2) Miogypsinoides complanatus / Nonion granosus Assemblage Zone:
This assemblage zone overlies a barren interval and the thin nummulitic coquina bed of the Umm Ad Dahiy Formation and underlies the nummulitic coquina horizon of the Bu Hashish Formation. It is encountered in the lower two third of the Bu Hashish Formation (Figs.3, 5), and includes the stratigraphic interval from sample no. 45 to 51. This zone is characterized by a flood of larger and smaller benthonic foraminiferal species, the most frequent taxa are: Miogypsinoides complanatus SCHLUMBERGER, Miogypsinoides sp., Miogypsinoides mediterranea DROOGER, Operculina carpenteri DEFRANCE, Operculina complanata DEFRANCE, Miogypsina intermedia DROOGER, Lepidocyclina sp, Nummulites fichteli MICHELLOTTI, Nummulites contortus (reworked), Nummulites beaumonti D’ ARCHIAC, Discordes obtusus D'ORBIGNY, Eponides sp., Rosalina obrotula (TERQUEM), Cibicides pseudoungerianus (CUSHMAN), Lenticulina subpapillosus (NATUALL), Textularia schencki (CUSHMAN & VALENTINE), Nodosaria capitata BOLLI, Nodosaria consobrina (D'ORBIGNY), Cibicides lobatulus (WALKER & JACOB), Cyclimina sp., Lagena ciperoensis (CUSHMAN & STAINFORTH), Spiroplectimmina trinidontensis (CUSHMAN & RENZ) Bolivina advena CUSHMAN, Bolivina budensis (HANTKEN), Cibicids micrus BERMUDEZ, Chilostomella cyclindeoides (REUSS), Stilostomella adolphina D'ORBIGNY, Nonion granosus D'ORBIGNY, Nonion boueanum D'ORBIGNY, Nonion scaphum (FICHTEL & MOLL) and Marginulina sp. This association of larger and smaller benthonic foraminifera indicates Late Oligocene (Chattian) age.

The argillaceous limestone interbeds showed a poor content of planktonic foraminifera: Globorotalia kugleri BOLLI, Globorotalia siakensis LE ROY, Globorotalia obesa BOLLI, Globorotalia increbenscens BANDY, Cassigerinella chipolensis BOLLI, Globigerina ciperoensis angustiumbilicata BOLLI, Globigerina praebulloides BLOW, Globigerina woodi JENKINS, Catapsydrax dissimilis (CUSHMAN & BERMUDEZ). This association assigned the Bu Hashish Formation to Late Oligocene age (Chattian). The Miogypsinoides complanatus / Nonion granosus Assemblage Zone could be correlated with the Late Oligocene Globigerina ciperoensis ciperoensis and Globorotalia kugleri Zones (P22/N3) of BLOW (1969,1979) and BOLLI & SAUNDERS (1985). Therefore, the absence of Globigerina ampliapertura BOLLI and Globorotalia opima opima BOLLI, which are index planktonic foraminiferal species for the Middle Oligocene age, strongly suggest unconformable boundary between the Umm Ad Dahiy Formation and the overlying Bu Hashish Formation. This unconformable contact is also supported lithologically by the existence of argillaceous sandstone and thin nummulitic coquina bed (paralic to littoral lithofacies) at the uppermost part of Umm Ad Dahiy Formation.

3) Zone with abundant Nummulites spp.:
This zone is encountered in the upper part of Bu Hashish Formation, covering the stratigraphic interval from sample 52 to 53 (1.5 m in thickness) and characterized by low diversity and high frequency of larger benthonic foraminiferal species especially Nummulites spp. The most common species are Nummulites fichteli MICHELLOTTI, Nummulites vercus JOLY & LEYMERIE, Nummulites vascus LEYMERIE, Nummulites beaumonti D’ARCHIAC, Nummulites fabiani PREVER (reworked), Nummulites striatus (BRUG), Nummulites contortus (reworked), Nummulites variolarius LAMARCK, Heterostegina antilles CUSHMAN, Heterostegina sp, Lepidocyclina sp., Nummulites intermedius CUSHMAN, Nummulites bouillei D’ARCHIAC, Assilina sp., and Ammonia beccarii LINNÉ. This association indicates Late Oligocene (Chattian) age for the Bu Hashish Formation.

This nummulitic coquina indicates an accumulation in a paralic environment. This interpretation is strengthend by the overlying 1m thick of gypsum bed, which reflects a deposition in hypersaline lagoon. Consequently, these facies indicate fluctuations in the sea level within the uppermost part of Bu Hashish Formation (regressive phase)


ECOLOGICAL SIGNIFICANCE OF IMPORTANT BENTHONIC FORAMINIFERA

Fifty-one benthonic foraminiferal species have been recorded from the Oligocene sequence of Gabal Bu Husah; eleven of these taxa are quite abundant along the succession. The distribution of these abundant species as well as the planktonic / benthonic ratio are presented in figures.4 - 6. The lithologic criteria and the microfacies analysis combined with the distribution of the foraminiferal species, can be used to show the changing in the paleoenvironmental conditions during the deposition of the Oligocene sequence within the uppermost part of Bu Hashish Formation. The paleoecology of these eleven species are briefly discussed:

1- Ammonia beccarii (LINNÉ): is abundant in the middle part of the Oligocene sequence and quite rare in the lower and uppermost part of the section. It is mainly found in the sandy limestone beds that contain also larger benthonic foraminifera. MURRAY (1968, 1973,1991) reported that this species lives in hypersaline lagoons and estuaries (brackish to hypersaline environments) and in temperature ranged from 15° - 30° C as well as common in depth of 0 – 50 m. POAG (1981) and WALTON & SLOAN (1990) stated that Ammonia beccarii (LINNÉ) is typical dwellers of the shallow waters and it withstands changing in temperature between 0° - 50° C and also salinity changes from 1 to 90 %o. The smaller and thinly calcareous specimens of this species in the present study are pointing hypersalinity and abundant food supply with depths ranged from 10 - 50 m (inner shelf environments).

2- Cibicides lobulatus (WALKER & JACOB): is relatively frequent in the Miogypsinoides complanatus / Nonion granosus zone. In addition, fewer specimens may occur in some samples of the Elphidium minutum zone. MURRAY (1973) & LUTZE (1980) and SNYDER et al. (1988) mentioned that this species lives as an epiphyte (fixed on solid substrate) and tolerant of increased salinity in water depths rarely up to 80 m (inner shelf environments) preferring well oxygenated conditions. Its association with Ammonia beccarii (LINNÉ) also indicates moderate water depth.

3- Elphidium spp.: are commonly abundant at the base of the Oligocene sequence in Gabal Bu Husah, especially in the Elphidium minutum zone. Elphidium crispum (LINNE’) and Elphidium advenum (CUSHMAN) are commonly occurs in the middle part of the previous zone. MURRAY (1968 a, b, 1973 &1991) reported that Elphidium minutum (REUSS) and Elphidium crispum (LINNÉ) along with Elphidium macellum (FICHTEL & MOLL) are indicative of the inner shelf (0 –50 m) and dominated in normal salinity as well as temperatures of more than 15°C. BANDY (1954), SAID (1950) & WALTON (1955), and VAN MORKHVEN et al. (1986) recorded Elphidium minutum (REUSS), Ammonia beccarii (LINNÉ), Cibicides boueanus D‘ORBIGNY and Quinqueloculina sp. generally thrive in water depths less than 90 m.

4- Amphistegina lessonii BRADY: dominates in the Nummulites spp. zone especially in samples nos. 52, 53 and it is relatively frequent in the Miogypsinoides complanatus / Nonion granosus zone. GALLOWAY (1933) and SOUAYA (1963) reported that the co - occurrence of the Amphistegina lessonii BRADY, Operculina spp, and Quinquloculina spp., are a good indicator of shallow warm marine environments. MURRAY (1973, 1976) also mentioned that the association with the Ammonia beccarii (LINNÉ) generally indicate shallow warm waters.

5- Siphonina reticulata (CZJZEK): is rare in the Oligocene sequence of Gabal Bu Hush. BERGGREN & HAQ (1976) described this species together with Siphonina planoconvexa, which is characteristic of outer neritic environments and even greater water depths. The co - occurrence of this species with the planktonic foraminifera in the studied argillaceous limestone samples suggests open marine environment (outer neritic zone).

6- Operculina spp.: is commonly found in the zone of Nummulites spp. and also in the Miogypsinoides complanatus / Nonion granosus zone. It is represented essentially by Operculina complanata DEFRANCE with Heterostegina spp. GALLOWAY (1933); LEMOINE (1940) and ADAMS (1965) mentioned that these species are good indicators of shallow warm marine environments. The association of these large benthonic foraminiferal species with Ammonia beccarii (LINNÉ) and Amphistigina lessonii BRADY suggests predominance of shallow water environments.


7- Bolivina spp.: is frequent in most samples of the Oligocene sequence in Gabal Bu Husah. It is recorded in the Elphidium minutum and Miogypsinoides complanatus / Nonion granosus zones but not in the Nummulites spp. zone. It is represented commonly by Bolivina arta D‘ORBIGNY, Bolivina advena CUSHMAN, Bolivina hebes SAID and Bolivina bundensis BOLLI. These species are commonly associated with the argillaceous limestone beds and predominantly occur with the planktonic foraminifera suggesting a relatively deep marine water environments (MURRAY, 1965, 1973 & BOLTOVSKOY, 1983).

8- Eponides spp.: is frequently found in the argillaceous limestone samples of the Umm Ad Dahiy Formation and represented mainly by three species Eponides boueanum D’ORBIGNY, Eponides schreibersii D’ORBIGNY and Eponides antillarum D’ORBIGNY. MURRAY (1973) mentioned that this genus is quite common in normal marine deposits and has a wide depth - range and temperature tolerance. In the present material, the association of these species with the planktonic foraminifera suggests a relatively open deep marine environments (outer neritic zone).

9- Nummulites spp.: are commonly coexist with larger benthonic foraminiferal species of Miogypsinoides spp. and Opeculina spp. and Lepidocyclina spp. as in situ or reworked broken fragments along with pelecypods and echinoderms fragments forming the nummulitic coquina bed in the middle and upper parts of the studied Oligocene sequence. Among the common Nummulites spp. recorded are Nummulites budensis HANTKEN, Nummulites fabianii (PREVER), Nummulites contortus (reworked from Late Eocene sediments), Nummulites beamounti D’ARCHIAC, Nummulites bullatus AZZAROLI, and Nummulites rotularius DESHAYES. These species are dominated in the zone with abundant Nummulites spp. denoting littoral to paralic conditions.

10- Nonion boueanum D’ORBIGNY / Nonion granosus D’ORBIGNY: are commonly recorded in the in the studied Oligocene materials and being represented in the two rock units. They are mainly a species of middle neritic zone and reached to the outer zone (BERGGREN & HAQ, 1976) but additionally occurs in inner shelf areas and even in bays and estuaries (POAG,1981). Their recent distribution reaches from depths of 30 m to 200 m in NW Africa. This genus prefers oxygen - rich conditions (SNYDER et al., 1988).

11- Miogypsina spp.: GALLOWAY (1933), LEMOINE (1940) and DROOGER & MAGNE (1959) reported that Miogypsina spp. indicates warm shallow marine environments with sandy sediments. BANDY (1960) recorded this genus from the restricted warm marine water up to 40 m deep with temperature around 25º C. In most of the Bu Hashish Formation samples, this genus always accompanied by other miliolids and bryozoa as well as rotaliids and coralline red algae (Lithophyllum spp., Lithothamnium spp. and Lithoporella spp.)


FIELD AND MICROFACIES STUDIES

The following is a discussion of the field studies, petrography and main microfacies types characterizing the marine Oligocene sequence at Gabal Bu Husah. The classification of DUNHAM (1962), FOLK (1956, 1959, 1962 & 1974) and FLÜGEL (1982) are adopted for the carbonate rocks. While the classification of PETTIJOHN (1974) for the clastic rocks is followed. About eleven microfacies types are recognized, nine of them belonged to the carbonate rocks and the other two microfacies attributed to the clastic rocks. The Umm Ad Dahiy and Bu Hashish formations are discussed from base to top:


A- Microfacies types of the Umm Ad Dahiy Formation:

1) Sandy biosparite facies ( bioclastic packstone / grainstone facies):
This facies is recorded in the lowermost part of Umm Ad Dahiy Formation. It is characterised in the field by its creamy white in colour, massive, hard, fossiliferous, sandy limestone beds, conformably overlies the late Eocene Thamat Formation (Fig.3). The microscopic inspection revealed that the framework is composed of bioclastic grains (70%) in the form of bivalved shell fragments, echinoid spines, bryozoa fragments, benthonic and planktonic foraminiferal tests. The non-skeletal grains (30%) are represented by subangular to subrounded, moderately sorted quartz grains and subrounded carbonate intraclasts of micrite composition embedded in sparry calcite cement (Pl.2, Figs.5 & 6). This facies type reflects deposition in shallow marine environment (inner neritic zone).

2) Foraminiferal biomicrite (foraminiferal wackestone) facies:
This facies is represented by greyish white, massive, fossiliferous, argillaceous limestone interbedded with the dolostone and sandy dolomitic limestone beds. It is 0.5 - 1.5 m thick. The microscopic investigation showed this facies is composed of minute skeletal constituents in the form of benthonic and planktonic foraminifers (60% of the framework material). The non-skeletal constituents (40%) are represented by subangular quartz grains and carbonate intraclasts of fine-sand size (Pl. 2, Fig.4). The framework material is embedded in microcrystalline sparite (micrite) matrix that diagenetically changed to microspars (Aggrading neomorphism). This type of facies reflects deposition in relatively open deep marine environments (middle to outer neritic zones).

3) Dolostone Facies (dolomitic packstone facies):
This facies is commonly recorded in the Umm Ad Dahiy Formation. It is characterized in the field by its greyish yellow, hard, massive, fossiliferous dolostone beds (2m to 5m in thickness). The petrographic investigations showed that this facies is characterized by abundant, minute benthonic foraminiferal species Bolivina spp, Cibicides spp. and Nonion spp., constituting about 40% of the bulk of this facies. Molluscan shell fragments, echinoide spines, sponge spicules (20%), are present. The intraclasts are less in abundant in the form of minute quartz grains. The cement is represented by micrite (microcrystalline sparry calcite) that partially dolomitized (Pl.3, Fig.6). Most of the dolomite rhombs exhibit distinct cloudy zone i.e. dark core and clear outer rim.

4) Sandy intrabiosparite Facies (intraclastic packestone / grainstone ):
This is the dominant facies of Umm Ad Dahiy Formation and ranges in thickness from 2 to 6m in the field. It is yellowish white in colour, compact, fossiliferous with oyster shell fragments, sandy limestone interbedded with the foraminiferal biomicrite facies. The framework is composed of skeletal and non-skeletal constituents. The skeletal components (70%), are represented by molluscan shell fragments, larger benthonic foraminifera Miliolide sp., reworked Nummulites sp., Operculina sp. and Textularia sp. The other biogenic constituents are echinoid spines, bryozoan fragments and poorly preserved coralline red algae occur. The non-skeletal components are represented by extraclasts of angular to subangular, poorly sorted chert clasts and quartz grains as well as subrounded carbonate interaclasts of micrite composition. These skeletal and non-skeletal grains are embedded in coarse sparry calcite cement (Pl.2, Figs.2&3). This type of facies reflects deposition under shallow warm marine environment (inner neritic zone).



5) Oolitic grainstone facies:
This facies is recorded overlying the dolomitic facies and distinguished by its yellowish white colour, porous, fossiliferous oolitic limestone is about 1m. This facies is composed mainly of ooides of clear nucleated core quartz grains or echinoide spine and thick dark micritic rim, well rounded, and subordinate bioclasts in the form of benthonic foraminifers, shell fragments and echinoide spines. These allochems are presented in sparite cement denoting a shallow marine, agitated water environment (BATHURST, 1975;WILSON, 1975).

6) Argillaceous litharenite facies:
This facies is predominantly located in the uppermost part of the Umm Ad Dahiy Formation and consists commonly of cross-bedded argillaceous sandstones intercalated with some thin bands of gypsiferous clays. This facies contains abundant grains of quartz and chert clasts. The quartz grains are subangular to subrounded, poorly to moderately sorted, submature. The chert intraclasts are subangular to subrounded poorly sorted, derived from chert bands in the late Eocene Thamat Formation. Bioclasts are represented by shell fragments, bryozoans. These components are embedded in argillaceous matrix. This type of facies indicates deposition in fluviomarine environments as evident by the presence of organic detritus such as gastropods, echinoid spines, bryozoans and inoceramids.

B- Microfacies types of the Bu Hashish Formation:

1) Calcareous sublitharenite facies:
This facies in the field by forms a thick bed (2-4m). It is represented by yellowish white to yellow, hard, massive, medium to fine grained, moderately sorted calcareous sandstone, moderately or slightly fossiliferous with Pecten spp. and oyster shell fragments. This facies consists essentially of subangular to subrounded, moderately sorted, submature, detrital quartz grains of monocrystalline type (65% of the framework) (Pl.1, Fig. ). Rock fragments (25%) existed in the form of angular to subangular chert clasts and subrounded dark terrigenous carbonate lithoclasts of medium sand sizes and of dark micrite composition with weathered glauconitic pellets. Feldspars are also present (10%). The framework material of this facies is cemented by dark microcrystalline calcite (micrite) showing signs of corrosion at the boundaries of the quartz grain (Pl.3, Figs.1&2) forming engulfed clasts. this type of facies is most probably deposited under shallow marine environments (inner neritic environments) with synchronous supply of lime mud.

2)Argillaceous foraminiferal biomicrite facies (wackestone / packstone facies):
This facies is recorded overlying the previous facies and represented in the field by a pale grey to dark yellowish grey, thinly bedded, compact, partly gypsiferous, argillaceous limestone, 0.5-1m in thick, with abundant benthonic and planktonic foraminifera. This facies consists predominantly of benthonic foraminifers such as Gumbelina spp, Bolivina spp and rare planktonic forams in the form of Globigerina spp. (Pl. 3, Fig. 3) and constituting about 65% of the framework. The other bioclasts are depicted in the presence of echinoid spine, fragmented foraminiferal tests and molluscan shell fragments (15% of the framework). The non-skeletal components are represented by silt-sized quartz grains embedded in a matrix of microcrystalline micrite with argillaceous material. This micrite matrix is replaced in some places by recrystallized microspars (Aggrading neomorphism).The presence of micrite matrix and the high benthonic/planktonic ratio reflects deposition under open marine environments (outer neritic zone).



3) Dolomitic sandy biosparite facies (grainstone facies):
This facies is represented in the field by a yellowish white to creamy white, massive, sandy dolomitic limestone, abundant with larger benthonic forams and bivalve shell fragments. It attains 2-4 m thickness. This facies is composed mainly of bivalved shell fragments (molluscan shells 50%) and larger benthonic foraminifera such as Heterostegina spp, Nummulites spp (10%). The other bioclasts are represented by reworked coral fragments, skeletal coralline red algae, echinoid spines, bryozoa and ostracod tests (10%). The non-skeletal constituents (intraclasts) are extraclasts of angular to subrounded poorly sorted microcrystalline quartz grains, which are corroded by the sparite cement, forming about (20%). Subrounded carbonate intraclasts of micrite composition with subordinate rounded phosphatic grains of fine sand sized constitute about (10% of the framework). The skeletal and non-skeletal constituents are embedded in sparry calcite cement (Pl.3, Figs.4&5). Two main diagenetic processes affected this facies, 1) An intra - and intergranular cementation and existed in the filling of gastropod shells and fixed the framework material. 2) Aggrading neomorphism and depicted in the form of recrystallization of some shell fragments into a coarse granular sparry calcite with micritic envelope. This facies reflects deposition in shallow warm marine environments, most probably inner neritic zone.

4) Operculinid sandy biosparite facies (operculinid packstone / grainstone facies):
This facies is represented in the field by a yellowish white to creamy white, hard, massive, sandy limestone, fossiliferous (2m) with Operculinids and Heterosteginids. The petrographic studies showed that this facies is composed partly of larger benthonic foraminifera such as Operculina spp, Heterostegina spp (Pl.3 ,Fig.6) and some reworked Nummulites spp. (making up about 40% of the framework). The other biogenic fragments are represented by bryozoans, coral fragments, echinoid spines, molluscan shell fragments (30%). The lithoclasts are less frequent than biogenic constituents and represented by subrounded, medium - sized, moderately sorted terrigenous quartz grains (20%) and angular chert clasts of granule sizes with carbonate grains of dark micrite composition (10%). These skeletal and non-skeletal constituents are embedded in a coarse sparry calcite cement.

5) Nummulitic coquina bed (nummulitic grainstone / boundstone facies):
This facies is encountered in the upper part of the Bu Hashish Formation. The nummulitic coquina, attains a thickness of 2m thick and composed mainly of Nummulites spp. of variable size associated with Operculina spp., Lepidocyclina spp. and subordinate macrofouna in form of Balanus sp, oyster shells embedded in a yellow soft marl (Pl.2, Fig. 3). The great accumulation of such biogenic constituents indicates a shallow marine environment (littoral environment) with high-energy waves. This facies indicates also regressive phase as supported by the overlying gypsum bed.


CONCLUSIONS

The Oligocene sequence exposed at Gabal Bu Husah (NW Marada Oasis, south of Sirt Basin, Libya) conformably overlies the Late Eocene Thamat Formation and unconformably underlies the Early to Middle Miocene Marada Formation (Qarat Jahannam Member). Two main rock units, from base to top: 1) Umm Ad Dahiy Formation (Early Oligocene, Rupelian). and 2) Bu Hashish Formation (Late Oligocene, Chattian).

The Oligocene sequence yields a rich smaller and larger benthonic foraminiferal assemblages that permitted to identify about fifty one benthonic foraminiferal species. The foraminiferal analysis allowed to subdivide the sequence into three main local benthonic foraminiferal assemblage zone, arranged from base to top: 1- Elphidium minutum zone, included the middle part of Umm Ad Dahiy Formation. 2- Miogypsina complanatus / Nonion granosus assemblage zone, and 3- Zone with abundant Nummulites spp., that encompassed the Bu Hashish Formation. The carefull examination of the planktonic foraminiferal content associated with the argillaceous limestone interbeds revealed the presence of two planktonic foraminiferal zones , from base to top: 1- Cassigerinella chipolensis / Pseudohastigerina micra Zone of Early Oligocene age and 2 - Globigerina ciperoensis ciperoensis and Globorotalia kugleri Zones of Late Oligocene age. The absence of index planktonic foraminiferal species for the Middle Oligocene such as Globigerina ampliapertura BOLLI and Globorotalia opima opima BOLLI strongly suggested an unconformable boundary between the Umm Ad Dahiy Formation and the overlying Bu Hashish Formation. This unconformable contact is also supported lithologically by the existence of argillaceous sandstone and thin nummulitic coquina bed (paralic to littoral lithofacies) at the uppermost part of Umm Ad Dahiy Formation.


Eleven microfacies types were recognized covering the Oligocene sequence. These microfacies types clearly showed that Umm Ad Dahiy Formation was deposited under shallow warm marine conditions in its lower part but the conditions changed to fluviomarine in its uppermost part. While the Bu Hashish Formation was deposited in shallow marine (inner shelf environments) with probable hiatus in its middle part as evident by the presence of nummulitic coquina and gypsum beds (lagoonal to hypersaline environment). Returning to a shallow environment at the end of Oligocene time where the marine sedimentation ended.



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Fig. 1. Location map showing the study area , Gabal Bu Huash, NW Marada Oasis, Libya.
Fig. 2. Simplified geological map showing the Tertiary rock units in the study area, Marada Oasis, Libya.
Fig. 3. Lithologic description and distribution of macrofauna in the Oligocene Sequence of Gabal Bu Husah, Marada Oasis, Libya.
Fig. 4. Distribution range chart of the most common benthonic foraminiferal species recorded in the Oligocene Sequence of Gabal Bu Husah, Marada Oasis, Libya.
Fig. 5. Stratigraphic surface section of the marine Oligocene sequence at Gabal Bu Husah, showing the main microfacies types, depositional environments and main benthonic and planktonic foraminiferal zones, Marada Oasis, Libya.
Fig. 6. Planktonic / benthonic foraminiferal ratio and species diversity ( species richness) in the Oligocene sequence of Gabal Bu Husah, Marada Oasis, Libya.
Table. 1. Tentative correlation chart of the different Tertiary rock units as proposed by different authors ( modified after Mastera, 1985).























PLATE 1
(All the figured specimens are magnified as x = 50, unless otherwise mentioned)
Fig.1. Elphidium crispum (LINNE’), side view, sample 37. Fig. 2: Elphidium minutum (REUSS), side view, sample 38. Fig. 3. Elphidium advenum (CUSHMAN), side view, sample 34. Fig.4. Anomalinoides sp.,side view, sample 50, x60. Fig.5. Eponides sp., ventral view, sample 39. Fig. 6. Nonion granosus D’ORBIGNY, side view, sample 49. Fig.7. Haplophrgmoides sp. side view, sample 37. Fig. 8. Ammobaculites reophaceoides BARTENSTEIN, side view, sample 36. Fig.9. Textularia sp., side view, sample 39. Fig. 10. Dorothia sp, side view, sample 37. Figs.11,12. Textularia anglica CUSHMAN, sample 38. Fig. 13. Lagena sp., sample 47. Fig. 14. Uvigerina dumblei D’ORBIGNY, sample 51, x60. Fig. 15. Nodosaria sp., sample 48, x30. Fig. 16. Bolivina dilatata REUSS, sample 39, x60. Fig.17. Bolivina hebes (MACFAYDEN), sample 38. Fig. 18. Textularia schencki (CUSHMAN & VALENTINE), sample 49. Fig. 19. Spiroplectimina sp. sample 40. Fig. 20. Uvigerina sp., sample 37. Fig. 21. Quinqueloculina sp., sample 37. Fig. 22. Oolina lineata (WILLIAMSON), sample 42. Fig. 23. Ammonia beccarii (LINNE’), sample 38, x100. Fig. 24. Discorbis obtusus (D’ORBIGNY), sample 35, x100. Figs. 25,26. Operculina complanata DEFRANCE, sample 45, x30. Fig. 27. Nummulites sp., sample 53, x30. Fig.28. Eponides sp., sample 42. Fig. 29. Cibicides sp., sample 36, x40. Fig. 30. Ammonia sp., sample 40, x30. Fig. 31. Amphestegina lessonii BRADY, sample 54, x40. Fig. 32. Planulina sp., sample 51. Fig. 33. Lenticulina sp., sample 52, x30. Fig. 34. Globigerina ciperoensis ciperoensis BOLLI, umbilical view, sample 51. Fig. 35. Globigerina ciperoensis angustiumbilicata BOLLI, umbilical view, sample 51. Fig. 36. Globorotalia siakensis LE ROY, umbilical view, sample 51. Fig. 37. Cassigerinella chipolensis CUSHMAN & PONTON, sample 40.




PLATE 2

Fig.1. The northern escarpement of Gabal Bu Hashish with a prominent nummulitic coquina bed overlying the argillaceous limestone and sandy dolomitic beds.
Figs. 2,3. Sandy intrabiosparite facies (bioclastic packstone / grainstone facies):
Fig. 2. Subrounded quartz grains (Qz), benthonic foraminifera (Textularia sp.) (T) and fragmented larger forams. Fig. 3. Partially silicified shell fragment (SH) and subangular chert clasts (CH). in microsparite cement. Umm Ad Dahiy Formation, samples 36, 37, x60.
Fig.4. Foraminiferal biomicrite facies (foraminiferal wackestone):
Large number of benthonic foraminiferal tests with reworked Gumbellina sp.(G) and Globotruncana sp (GT). in microcrystalline micrite matrix. Umm Ad Dahiy Formation, sample No.33, x60.
Figs.5,6. Sandy biosparite facies (bioclastic grainstone):
Fig. 5. Subangular to subrounded, quartz grains (Qz), reworked oncolite grain with quartz as nucleus (O), and gastropod test filled with micrite matrix (G) embedded in coarse sparry calcite cement. Umm Ad Dahiy Formation, x60, sample No.32.
Fig.6. Subangular quartz grains (Qz), bivalve shell fragment (BS) in coarse sparry calcite cement.
Fig.7. Dolomite facies (dolomitic facies):
Dark nucleated core and clear outer rim of dolomite rhombs. Umm Ad Dahiy Formation, sample No. 34, x60.

Fig.8. Oosparite facies (oolitic grainstone facies):
Rounded ooids grains with quartz nuclei and shell fragments in sparry calcite cement. Umm Ad Dahiy Formation, sample No. 42, x80.







PLATE 3

Figs.1,2: Calcareous sublitharenite facies:
1: Subrounded to rounded, moderately sorted quartz grains (Qz), and badly preserved coralline algae (Ag) in dolomitic sparry calcite cement. Bu Hashish Formation, sample No. 46, O.L., X80. N.B. The dolomite rhombs with dark nucleated core.
2: Engulfed quartz grain corroted by the sparite cement (EG) with other subrounded grains. Bu Hashish Formation, sample No. 47, x80.
Fig 3. Argillaceous foraminiferal biomicrite facies (foraminiferal wackestone):
Planktonic (P) and benthonic (B) foraminiferal tests filled with intragranular sparry calcite, embedded in argillaceous micrite matrix. Bu Hashish Formation, sample No.51, x60.
Figs.4,5. Dolomitic sandy biosparite facies (bioclastic packstone / grainstone):
4. Subangular quartz grain (Qz), and subrounded carbonate intraclasts (IC) with planktonic foraminifers (P) embedded in intergranular sparite calcite cement. Bu Hashish Formation, sample No. 49, x60. Dolomite rhombs are very small in sizes and can not be distinguished in the photograph
5. large echinoid plate with dark cloudy center (E) and planktonic foraminiferal test (P) as well as dark carbonate intercalats in dolomitic sparry calcite cement. Bu Hashish Formation, sample No. 49, x60.
Fig.6. Operculinid sandy biosparite facies (operculinid grainstone ):
Large benthonic Heterostegina sp. (H) and Amphistegina sp. (A) with rounded echinoid spine in sparry calcite cement. Bu Hashish Formation, sample No. 50, x80.
Fig 7. Dolomicrite facies (dolomitic facies):
Small dolomite rhombs with a dark core and clear rims in micrite matrix. Umm Ad Dahiy Formation, sample No. 41, x60.