Author Topic: Depositional and Diagenetic History of the Neoproterozoic Silicified Stromatolit  (Read 1692 times)

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Author : Taj, R. J.; Mesaed, A. A
International Journal of Scientific & Engineering Research Volume 2, Issue 6, June-2011
ISSN 2229-5518
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Abstract-The Neoproterozoic stromatolitic carbonates of wadi Girshah, Saudi Arabia, is present in the middle part of the volcano-sedimentary succession of Ablah Group in Asir terrain. This succession is composed from rhythmic alternations of basic and acidic volcanic and related volcaniclastics with subordinated dolostones. It represents a shallow marine/non marine facies related to basic and acidic edifices during intermittent periods of volcanism, tectonism and sedimentation. The stromatolitic carbonates are present in the middle and topmost part of small-scale cycles begins by black tuffaceous mudstone which becomes upward slightly to highly dolomitized and/or calcitized giving-rise to black carbonate beds. The stromatolitic carbonate beds represent short-lived periods of cessation of volcanism and related volcanicalstic sedimentation. During these periods, the deposited basic tuffs become subjected to syn-sedimentary and diagenetic alterations leading to the formation of the carbonate beds. 
The detailed field measurements and meg- and microscopic description of the stromatolitic carbonates led to the recognition of two main types and related subtypes: laminated stromatolitic carbonates (type I and related sub-types IA, IB, IC) and banded stromatolitic carbonates (type II and related sub-types IIA, IIB). The main syn-sedimentary and diagenetic processes that involved in the petrographic and mineralogical evolution of the different types of stromatolitic carbonates are: ash fall out of basic tuffs along the sea floor during periods of cessation of volcanic activities, the biogenic degradation and devitrification of the deposited volcanic ash, calcitization, dolomitization and /or hematitization of the degraded tuffs. In ultimate stages of shoaling and cessation of ash fall out, intensive dolomitization of degraded tuffs and or Mg-rich lime mud led to the deposition of dolostones in the upper parts of the stromatolite-bearing cycles.
Index Terms-Arabian Shield stromatolites, Asir Terrain, Ablah area, Wadi Raniyah, Wadi Girshah, Precambrian stromatolitic carbonates, basic tuffs.

1  INTRODUCTION
THERE is abundant evidence that the first several billion years of life on Earth was microbial, and Precambrian carbonates are well-known for organo-sedimentary structures such as stromatolites. The oldest preserved stromatolitic carbonate platform is the 3.0 Ga White Mfolozi Formation of the Pongola Supergroup of South Africa [1].  The earliest stromatolite of confirmed microbial origin dates to2.724 billion years ago [2].

A recent discovery provides strong evidence of microbial stromatolites extending as far back as 3,450 million years ago [3], [4].  Studies of the early evolution of life and its contribution to the Archaean sedimentary record suffer from a scarcity of preserved microbial remains [5]. Stromatolites are fossils that are the result of the work of simple blue-green “algae” or Cyanophytes, which lived in chains or mats covered in a jelly like substance. By taking in carbon dioxide as a food source, they precipitate limy deposits on the jelly that builds up in layers. Widespread microbial mat growth on Precambrian sediment surfaces, whether carbonate or siliciclastic, is widely accepted [6], [7].

The interplay of biological and environmental factors produced widespread, diverse and abundant forms of stromatolites that have varied over time, with particular forms restricted to certain time intervals [8]. The stromatolites are used for stratigraphic purposes. Stromatolites were originally formed largely through in situ precipitation of laminae during Archean and older Proterozoic times, but that younger Proterozoic stromatolites grew largely through the accretion of carbonate sediments, most likely through the physical process of microbial trapping and binding [9]. Microfossils from the Lower Kundelungu (Late Precambrian) of Zambia has been described [10]. The microbial mat-related structures on the basis of the processes involved, such as growth, metabolism, destruction, decay, and diagenesis [11]. His scheme appreciates the difference in the nature of proxy records of past proliferation of microbial mats between sandstones and mudstones. The number of recognized columnar stromatolite forms rises to a maximum in the Mesoproterozoic, before dropping sharply in the Neoproterozoic, with relatively few in the Cambrian [12].

2. GEOLOGIC SETTING
The present paper aims to though light upon the depositional and diagenetic history of the stromatolitic carbonates of Wadi Girshah, Ablah District. The Ablah group [13] is present in the southwestern part of the Arabian shield. The term Ablah Formation was firstly introduced to describe the thick volcano-sedimentary succession which is entrapped in N-S trending graben. This succession is bounded from the west by the N-S trended sheered volcano sedimentary succession of W. Shwas belt (Fig. 1). [14] changed the stratigraphic and geographic limits of the Ablah group in his map of the Arabian Shield. He restricted the Ablah group to the volcanic and sedimentary rocks in the vicinity of the Ablah prospect and described the succession of Ablah Formation under the term Ablah group (Cryogenian- Ediacarian layered rocks).
[15], [16], [17], [18] studied the economic potentiality of many  mineralized areas within Ablah terrain.
The succession of Ablah district was subdivided by [19] into the following rock units which are arranged from base to top into: Girshah andesite, Khutnah Formation, Jerub Formation and Ablah Formation. The Ablah Formation was further subdivided by Zakir (op. cit.) into the followings: lower green unit, upper green unit, lower red unit, siltstone-dolostone unit, and upper red unit which is further subdivided into lower, middle and upper member.
The Neoproterozoic volcano-sedimentary succession of Wadi Girshah area has been assigned by [20] to be consists of three main units and represents gradation from inner shelf, shallow marine, lacustrine delta and fluvio-lacustrine environments.They also concluded that, this succession was formed during intermittent periods of volcanism; tectonism and sedimentation within inter arc-, back arc-, and intra arc- depositional settings. The lower unit of [20] consists mainly of basic volcanics and related green and red volcaniclastic beds. The middle unit is stromatolitic carbonate-bearing and it consists of acidic volcanics and intercalated dolostone (unit 2). They pointed out that, this unit represents a shallow marine/non marine facies related to felsic edifices and related to the formation of island arc. The dolostones represent a back arc situation of restricted starved and isolated marine environments of low volcaniclastic input. Rhyolite and rhyolitic tuffs support deposition in intra-arc grabens and back arc regions; continental extensional tectonic zones. The upper unit (Basic volcanics and related volcaniclastic green, red and yellow dolostone beds (unit 3). This unit was formed during a new intra-arc rift volcanism during an extensional regime, dominated by deposition of basic green and red volcaniclastics and dolodstones in shallow marine restricted environments.non marine facies related to felsic edifices and related to the formation of isl

In this study a new geologic map of wadi Girshah area were carried out (Fig. 2). According to this map, the study area comprises ten main units. These units are arranged from the older in the western part of the study area and the younger in the eastern part. They are disrubted by a series of major strick-slipe E-W faults and also by a series of double N-S plunged anticlines [20]. These units are described here as follows:

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