ГЕОХИМИЯ, 2003, № 5, с. 570-576
КРАТКИЕ СООБЩЕНИЯ
An Authigenesis of Glauconite in Association with Unconformity: Acase Study from the Albian/Cenomanian Boundary at Gabal Shabrawet, Egypt © 2003 r. H. A. Wanas
Geology Department, Faculty of Sciences, Menoufia University, Shebin El-Kom, Egypt
e-mail: wanas2000@yahoo.com Received September 6, 2001
INTRODUCTION
Glauconite is a specific name for a green mineral species of hydrated iron-rich micaceous clay mineral (illite in which 50% of the octahedron positions are replaced by iron, and have a relatively high K content) (McRae, 1972; Ireland et al., 1983). Furthermore, the term glaucony was suggested by Odin and Matter (1981) as a facies term. To avoid confusion, the term glaucolith has been also introduced for the individual glauconite pellet or grain (Madsen, 1983).
Authigenic glauconites can be developed in various substrates. The most common types of substrate are: (1) internal moulds of foraminifera and ostracods (Ehl-mann et al., 1963; Bjerkli and Ostmo-Saeter, 1973; Humphreys and Balsón, 1985; Lim et al., 2000); (2) carbonate shell fragments (Odin and Matter, 1981; Amireh et al., 1998); (3) mineral grains and rock fragments, for example, quartz, feldspar, biotite, musco-vite, calcite, phosphate, volcanic glass shards and chert grains (Ojakangas and Keller, 1964; Hein et al., 1974; Jeans et al., 1982; Odin, 1985; Odin and Mortin, 1988; Chaudhuri et al., 1994). Such variation in glauconite-bearing substrates explains why there is a wide variety of glauconite morphologies (Triplehorn, 1966; McRae, 1972; Odin and Fullagar, 1988; Odin and Morton, 1988).
Authigenic glauconites are reported in numerous studies to form in open-shallow marine environments far from zones of active sedimentation (e.g., Odin and Matter, 1981; Bornhold and Giresse, 1985; Humphreys and Balsón, 1985; Odin and Morton, 1988; Odin and Lamboy, 1988; Amorosi, 1997; Kelly and Webb, 1999). Recently, glauconites were found in a high energy environment with a high rate of sedimentation (Glenn and Arthur, 1990; Chaudhuri et al., 1994; Chafetz and Reid, 2000).
The possible formation of authigenic glauconites in association with unconformity is still little investigated (Krumbein, 1942; Mesolella, 1965; Khalifa, 1983). Therefore, the present work discusses the possibility of formation of authigenic glauconite in association with unconformity that separates between the Albian and Cenomanian rocks at Gabal Shabrawet, Egypt.
GEOLOGICAL SETTING
Gabal Shabrawet occurs in the northeastern portion of the Eastern Desert of Egypt between latitudes 30° 14' and 30° 17' N and longitudes 32° 15' and 32° 18' E (Fig. 1). It is a small asymmetrical plunging anticline striking northeast-southwest and plunging to the southwest (Fig. 1). This structure was affected by faults associated with the "Syrian Arc System".
At the core of Gabal Shabrawet anticline, the Lower Cretaceous succession crops out (Fig. 1). This succession dips steeply and is faulted up against gently dipping Eocene and younger rocks at the northeastern portion of Gabal Shabrawet. The Lower Cretaceous rocks are composed of alternating siliciclastic and carbonate facies. They are subdivided into three main sequences: lower regressive clastics, middle carbonate/clastics and upper carbonate-dominated sequences (El-Azabi, 1999). He added that the lower regressive clastics sequence belongs to the Malha Formation of Abdallah et al. (1963) and is of an Early Aptian age. The middle carbonate/clastics sequence and the upper carbonate-dominated sequence are equivalent to the Risan Aneiza Formation of Said (1971) and are of the Late Aptian/Albi-an age. The Risan Aneiza Formation is unconformably overlain by the Cenomanian carbonate rocks (Figs. 2,3). These Cenomanian carbonates belong to the lower carbonate sequence of El-Azaby (1999), and correlated with the Halal Formation of Said (1971).
The glauconite samples studied here were collected from the deposits associated with the unconformity that separates between the Upper Albian Risan Aneiza Formation and Lower Cenomanian Halal Formation at Gabal Shabrawet (Figs. 1, 2). The Upper Albian rocks are composed of argillaceous limestone with minor dolos-tone interbeds (Fig. 2). They are unconformably overlain by the Cenomanian carbonate rocks (Figs. 2, 3). These Cenomanian carbonates are made up of argillaceous to chalky limestone and dolostone (Fig. 2). Field observations showed that the glauconite occurs as yellowish green massive bed including basal pebbles of argillaceous limestone (Fig. 3), and has a thickness ranging from 20 to 55 cm.
30° 17'
30° 16'
1 1 1 Mediterranean Sea <g>
N, Gabal
^ V Shabrawet
Cairo
Western
Desert
/ Eastern
j Desert
1 s
I «)
0 50 100 km iii \ \
H Oligocene | | Recent EX] Upper Eocene ® ££ Middle Eocene
¡w
Turonian
X? Cenomanian Albian
Fault
Plunging anticline
Railway
/ Asphaltic / Road
Late Aptian
0 1 2 3 4km Earli Aptian i-1->-1-1
32°16' 32° 18'
Fig. 1. Geological map of Gabal Shabrawet (modified from El-Azaby, 1999).
On the basis of the criteria of Krumbein (1942) and Shanmugam (1988), the diagnostic criteria of unconformity herein represent by the occurrence of (i) an un-dulatory contact at the base of glauconite bed (Fig. 2) and 00 basal pebbles of the underlying argillaceous limestone in the glauconite bed (Fig. 2).
METHODS AND ANALYTICAL TECHNIQUES
The collected glauconite-bearing samples were thin-sectioned and investigated under the polarized microscope. To determine the morphology of glauconite, car-
bon-coated chips of glauconite-bearing samples were studied under scanning electron microscope (SEM, type JSM-840). Back scattered electron images (BSEI) analysis was employed to illustrate the texture and compositional variations of glauconites. Selected samples were subjected to X-ray diffractometer with Ni-fil-tered, Cu radiation (XRD, type Philips 1980) to identify the diagnostic peaks of glauconite. The chemical composition (major elements) of glauconite was determined by using a JEOL-JSM-840 electron probe mi-croanalyzer operated at 15 kV accelerating voltage and
Age Rock Unit Lithological Characters
Cenomanian ie Halal Formation Argillaceous to chalky limestones and dolostones. The limestones are rich in bivalve marine fauna.
H Glauconite in the unconformity.
Albian The Risan Anez; Formation Argillaceous limestones with few thin dolostone interbeds.
Fig. 2. Representative stratigraphie succession of the Albi-an/Cenomanian rocks at Gabal Shabrawet and their unconformable relationship.
1-2 m electron beam diameter. The resulted data were corrected by the method of ZAF.
PETROGRAPHICAL AND CHEMICAL CHARACTER OF GLAUCONITES
Thin-section petrography and back scattered electron images analyses showed that the studied glauco-nites are commonly present as streaks, patches and rugged peloids (Figs. 4a, 4b). They are green in color and commonly present as partial replacement of calcite
groundmass through its cleavage planes and cracks (Fig. 4a). Advanced stage of this replacement is traced by an occurrence of rugged peloids of glauconite having tiny remnants of calcite (Fig. 4b). SEM images and XRD analyses indicate that the studied glauconites are interstratified glauconitic smectite (Figs. 4c, 5) enclosing relics of feldspar (Fig. 4d).
The electron microprobe analysis of individual glau-coliths from different glauconite samples (table) showed that the chemical composition of the studied glauconites varies within the range values of other glauconites adopted by many authors in the International References (e.g., Odin and Matter, 1981; Hughes and Whitehead, 1987; Abu El-Hassan and Tichy, 2000), except for slightly higher iron (Fe3+) value (22.5-25.39%). The low iron-bearing glauconites were found in association with low Eh (reducing conditions) and during burial diagenesis (Ireland et al., 1983), whereas the high-iron bearing type was observed in recent sediments at the transition zone from oxygen-poor to oxygen-rich (where sea level has fallen and there was a break of deposition) (Odin, 1985). Consequently, the higher iron value of the studied glauconite may be related to an increase of iron during subaerial exposure (unconformity) in which oxidizing conditions were prevailed. Chemically, the evolution of glaucony (maturity) has been commonly divided into four stages: nascent (K20 = 2-4%), slightly evolved (K20 = 4-6%), evolved (K20 = 6-8%), and highly evolved (K20 > >8%) (Odin and Matter, 1981). Consequently, the studied glauconites are of either evolved or mature type. This is documented by the potassium content of glauconites (K20 6.79-7.84%, table).
Fig. 3. Field photograph at Gabal Shabrawet showing the glauconite cemented the basal conglomerate in the unconformity surface between the Halal Formation (A) and the Risan Aneiza Formation (B). Notice the argillaceous limestone pebbles (see arrows) within the basal conglomerate (C).
Fig. 4. (a) Photomicrograph in plane light showing glauconite streaks (arrows) and patches (g) within calcite groundmass. Note the glauconitization occurs along cleavage planes and cracks of calcite. (b) Photomicrograph in plane light showing an advanced stage of glauconitization within calcite. This is noticed by the development of glauconite patches to rugged grains (g) within calcite groundmass. (c) Aclose-up on a glauconite grain clarifying its highly smectitic composition, (d) Scanning electron image (SEM) showing a glauconite grain enclosing relics of feldspar (k).
GLAUCONITE AUTHIGENESIS: THEIR RECOGNITION
There are two origins of glaucony: authigenic (autochthonous) and allogenic (allochthonous), with a further division of allochthonous glaucony to paraautoch-thonous or intrasequential and detrital or extrasequen-tial (Amorosi, 1995). The autochthonous glaucony has not experienced significant transport from its place of formation, whereas paraautochthonous glaucony incl
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