JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.87, January 2016, pp.69-84
Coralline Algae and Benthic Foraminifera from the Long Formation (middle Miocene) of the Little Andaman Island, India: Biofacies Analysis, Systematics and Palaeoenvironmental Implications SUMAN SARKAR1*, AMIT K. GHOSH1 and G. M. NARASIMHA RAO2 1
Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow- 226 007, India 2 Department of Botany, Andhra University, Visakhapatnam – 530 003, India *Email:
[email protected]
Abstract: Biofacies analysis is used to understand the palaeoenvironmental implications of late middle Miocene (Serravallian) algal-foraminiferal assemblages and carbonate deposition pertaining to the Hut Bay section of the Little Andaman Island, India. This study is based on the novel dataset of coralline algae and benthic foraminifera evaluated from the thin section analysis. Systematic descriptions of the abundant algal-foraminiferal taxa have been provided. The Long Formation in the studied succession contains rich assemblages of coralline algae and benthic foraminifera with moderate abundance of corals and sparse occurrences of echinoderms, gastropods, molluscs, bryozoans and barnacles. Based on the biogenic sedimentary components, Little Andaman Island was positioned in a tropical shelf environment at this time. The depth gradient of algal assemblages shows affinity to numerous other carbonate settings globally. The fossil benthic communities are indicative of thriving in a meso-oligotrophic regime. Dominance of coralline algae and larger benthic foraminifera indicates deposition in upper photic zone to the upper part of the lower photic zone. Frequent alternations of lower-energy, mud-supported wackestones with higher-energy, grain-supported grainstones and packstones suggest the likely incidence of cyclones/storms during the depositional period. The facies gradients and high incidence of taphonomic signatures such as abrasion and fragmentation support the likelihood of parautochthonous to allochthonous deposition for the bulk of the succession. Keywords: Coralline algae, Benthic foraminifera, Biofacies, Little Andaman, Middle Miocene, Palaeoenvironment.
INTRODUCTION
Coralline red algae and benthic foraminifera are important biogenic components of Cenozoic as well as modern carbonate sediments worldwide (Hohenegger, 1995; Basso, 1998; Hohenegger et al. 1999; Braga and Aguirre, 2001; Rasser and Nebelsick, 2003; Rasser and Piller, 2004; Brandano et al. 2005; Kroeger et al. 2006; Bassi et al. 2009; Bassi and Nebelsick, 2010; Kundal and Kundal, 2010; Kundal et al. 2013, 2014; Martinus et al. 2013). In the context of the Indian subcontinent, the Andaman-Nicobar sedimentary basin has mainly been subject to studies focusing on algal diversity and taxonomy (Chatterji and Gururaja, 1972; Gururaja, 1977; Venkatachalapathy and Gururaja, 1984; Badve and Kundal, 1986, 1988, 1989, 1998; Chandra et al. 1999; Kundal and Wanjarwadkar, 2000, 2003; Ghosh et al. 2004; Saxena et al. 2005; Kundal, 2014; Sarkar and Ghosh, 2015) and general geology (Gee, 1927; Van Bemmelen, 1949; Rodolfo, 1969; Srinivasan, 1969, 1975, 1988; Sastry et al. 1973; Srinivasan and Singh, 1978; Sharma
and Srinivasan, 2007). Detailed biofacies and palaeoenvironmental studies of such relatively unexplored basins are, however, important for attaining sound information pertaining to regional palaeobiogeography, palaeoclimate and palaeoecology. The coralline red algae and benthic foraminifera recorded in this investigation originate from the Serravallian (late middle Miocene) carbonates of the Hut Bay region, located along the south-eastern coastline of the Little Andaman Island. Abundant assemblages of coralline red algae and benthic foraminifera in the Hut Bay limestones provide ample opportunities to study palaeoenvironmental controls on sedimentation operating in the tropical seascape. Benthic foraminifera respond to a combination of several interdependent factors including water depth and nutrient flux (Mutti and Hallock, 2003; Pomar et al. 2004). The relationship between coralline red algae and water depth has been demonstrated previously by several workers (Littler, 1973a, b; Van den Hoek et al. 1975; Adey et al.
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1982; Bosence, 1983; Braga and Aguirre, 2001; Kroeger et al. 2006). The apparent water depth dependency of coralline red algae has been interpreted to be mainly a consequence of their sensitivity to light (Adey et al. 1982; Minnery, 1990). More recent studies in eastern Australia using the revised taxonomy of Woelkerling (1988) have elaborated a depth zonation of several red algal taxa (Lund et al. 2000; Braga and Aguirre, 2004). The distribution of red algal subfamilies is not only distinct in deep and shallow-water deposits but also differs between tropical and temperate carbonate sediments (Braga and Aguirre, 2001). Since there has been no studies pertaining to the biofacies and palaeoenvironmental analyses of the Long Formation outcropping in the Hut Bay region of the Little Andaman Island, the principal objectives of this paper are (1) to describe the biofacies of the studied section pertaining to the Long Formation, (2) to present brief systematic description of the major coralline algae and benthic foraminifera, and (3) to decipher the depositional palaeoenvironment of the Long Formation. GEOLOGICAL SETTING
of the Hut Bay Quarry show dark grey to white mottling, algal oncoliths and a high fraction of vuggy and moldic porosity (Srinivasan and Chatterjee, 1981). MATERIAL AND METHODS
The late middle Miocene deposits were studied from the carbonate outcrop on Hut Bay coast. The main lithological feature of the section, about 30 m in thickness, is a fully lithified carbonate succession with largely a muddy matrix. Altogether thirty limestone samples (RHQ 1 to 30) were collected (Fig. 2) and four palaeontological thin sections from each sample (~3.5 x 5.0 cm) were prepared for the purpose of the biofacies analysis. Biofacies analyses with impetus on the carbonate textures were carried out according to Dunham (1962) and Flügel (2004). Microfossil assemblages were investigated in detail to obtain data for the interpretation of the depositional environment. Modern taxonomic concepts pertaining to neontological systematics (Rasser and Piller, 1999; Maneveldt et al. 2008) commonly used for coralline red algae have been applied in the present study. For algal identification, dimensions of cells and conceptacles were measured according to the latest method proposed by Aguirre and Braga (1998) and Rasser and Piller (1999). At least 20 cells of each cell type were
The Little Andaman Island (10º30’N - 10º54’N; 92º20’E - 92º37’E) is the southernmost island of the Andaman Group (Fig. 1). The coralline algae and larger benthic foraminifera analysed in the present study have been recovered from the samples belonging to the Long Formation encompassing carbonate rocks. This unit unconformably overlies a calcareous mudstone unit, and is exposed in a channel cut section, adjacent to Hut Bay Quarry No. 4. The Neogene stratigraphic sequence of the Little Andaman Island can be divided into seven chronostratigraphic stages based on the characteristic planktic foraminiferal zones (Srinivasan, 1978, 1988; Srinivasan and Singh, 1978; Sharma and Srinivasan, 2007) viz., Jarawaian (late early Miocene), Inglisian (early middle Miocene), Ongeian (late middle Miocene), Havelockian (early late Miocene), Neillian (late Miocene), Sawaian (early Pliocene) and Taipian (late Pliocene). The presently studied Long Formation of the Little Andaman Island belongs to the Ongeian Stage dating late middle Miocene (Srinivasan, 1988; Sharma and Srinivasan, 2007). Srinivasan (1975) distinguished two principal litho-units sharing an unconformable contact in the south-eastern part of the Little Andaman Island i.e., creamish yellow, molluscan rich limestone underlained by dark bluish grey, highly calcareous, soft and fragile mudstone. The limestones Fig. 1. Location of the study area in Andaman-Nicobar Islands, India.
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is followed. Taphonomy has been interpreted following Nebelsick and Bassi (2000), Basso et al. (2009) and Kundal (2011). Relative abundance of calcareous algae and benthic foraminifera were estimated in thin-section by image analysis and measuring the proportional area occupied by each taxon relative to the total biogenic population (Perrin et al. 1995). Digital photographs of the carbonate thin sections were taken with Olympus PM-20 exposure control unit mounted on Olympus BX 50 binocular microscope. RESULTS Facies Description
On the basis of the dominating biogenic components, texture and depositional fabric, six major facies types (MFTs) were identified: MFT 1 (Foraminiferal-Coralline Algal Grainstone), MFT 2 (Coralline Algal Wackestone), MFT 3 (Nummulitid-Orthophragminid GrainstonePackstone), MFT 4 (Coralline Algal-Orthophragminid Grainstone-Packstone), MFT 5 (Coralline AlgalForaminiferal Wackestone) and MFT 6 (AmphisteginaOperculina Grainstone). MFT 1: Foraminiferal-Coralline Algal Grainstone (Plate II, Fig. a)
Fig.2. Lithocolumn of the studied section with distribution of major facies types (MFTs).
measured (10 cells in case of poorly preserved specimens). Mean (M) and standard deviation (SD) were calculated. Cells of primigenous filaments were measured near the dorsal surface to avoid filament branching zones, which are difficult to recognise in thin sections. Cells of postigenous filaments were measured mostly in central thallus portions, due to the impact of maximum micritization in outer cell layers. In case of biofacies analysis, taxonomic uncertainties concerning fossil corallines as discussed by Braga and Aguirre (1995), Rasser and Piller (1999) and Bassi and Nebelsick (2000) are avoided by using generic names only. Standard literature sources have been referred for the taxonomic study of benthic foraminifera (Cushman, 1959; Boudagher-Fadel, 2008). Owing to the relatively poor preservation of the study material, taxonomic descriptions have been provided only for the specimens with fair degree of observable diagnostic features and minimum number of measurable cells. For biofacies analysis, the entire biogenic assemblages have been taken into consideration. Coralline algal growth-form terminology of Woelkerling et al. (1993) JOUR.GEOL.SOC.INDIA, VOL.87, JAN. 2016
This light to dark yellowish facies is dominated by nummulitids (20-25%) represented by Amphistegina, Operculina and some poorly preserved forms (probably Heterostegina), miogypsinoids (15-20%) represented by Miogypsinoides and some tests with poor preservation (possibly Miogypsina), other foraminifera (10-15%) represented by Carpenteria, Victoriella and geniculate coralline algae (20-25%) represented by Corallina and Amphiroa. A few mastophoroid forms represented by Spongites are also observed. Zooxanthellate corals (zcorals), abraded foraminifera, non-geniculate algal debris, echinoids, gastropods, molluscs and textulariids are the subordinate facies components. Grainstones show poor degree of sorting with the grains ranging in size from 0.75-2.1 mm. Fabric comprises of both oriented and chaotic setting of sediments with occasional presence of ripple marks. MFT 2: Coralline Algal Wackestone (Plate II, Fig. b)
This light greyish to dark yellowish facies is dominated by very much abraded and fragmented unidentifiable nongeniculate coralline algae (10-15%, probably mastophoroids), few fairly preserved Spongites and Lithophyllum (>5%), and some geniculate coralline algae represented by Corallina and Amphiroa (>5%). Due to
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excessive abrasion and fragmentation, generic diversity of several non-geniculate algal forms could not be ascertained. Smaller nummulitids, corals, textulariids, miliolids, rotaliids, echinoids, barnacles, peloids and molluscs are the subordinate facies components. Mud-supported wackestones present an asymmetric and chaotic fabric. MFT 3: Nummulitid-Orthophragminid Grainstone-Packstone (Plate II, Figs. c-d; Plate III, Fig. d)
This light to dark yellowish facies is dominated by nummulitids (15-20%) represented by Amphistegina, Operculina, Katacycloclypeus, few forms likely to be Spiroclypeus and Cycloclypeus, orthophragminids (15-20%) represented by species of Nephrolepidina and larger rotaliids (>5%) represented by Planorbulinella. Large melobesioids (Lithothmanion/Phymatolithon) are also observed (5-10%). Smaller rotaliids (possibly Pararotalia, Neorotalia, Cibicides, Elphidium), soritids (possibly Archaias, Amphisorus, Peneroplis, Sorites), fragments of geniculate coralline algae and non-geniculate rhodoliths, rare mastophoroids, corals, echinoids, barnacle shells, gastropods, molluscs and ooids are the components of subordinate facies. Poorly sorted grainstone-packstone facies are characterized by a packstone matrix. Size range of grains is 0.83-2.2 mm. Fabric is asymmetric and chaotic.
(5-10%) represented by Amphistegina and Operculina, other larger foraminifera (
