An abiotic model for the development of textures in some South Atlantic early Cretaceous lacustrine carbonates V. PAUL WRIGHT 1,2 * & ANDREW J. BARNETT 3 1 PW Carbonate Geoscience, 18 Llandennis Avenue, Cardiff CF23 6JG, UK 2 Natural Sciences, National Museum of Wales, Cardiff CF10 3NP, UK 3 BG Group, 100 Thames Valley Park, Reading RG6 1PT, UK *Corresponding author (e-mail: v.vpw@btopenworld.com) Abstract: The cyclic distribution of various types of carbonates and Mg-clays in early Cretaceous rift-sag phase lacustrine carbonates from the subsurface of the South Atlantic provides an insight into how evolving lake chemistries in highly alkaline settings control facies development. The typi- cally subdecametre scale symmetrical and asymmetrical cyclothems exhibit three main com- ponents: mud-grade laminated carbonates, millimetre-diameter spherulites with evidence of having been in a matrix of Mg-silicates, and millimetre–centimetre calcitic shrub-like growths. The laminites contain conspicuous numbers of ostracods and vertebrate remains and were pro- duced by short-lived pluvial events, causing expansion of shallow lakes. Later evaporation trig- gered Mg-silicate precipitation and calcite nucleation within gels to produce spherulitic textures. When the rate of gel precipitation decreased or ceased, calcite growth, now less inhibited, produced shrub-like calcites resembling those produced abiotically in modern travertines, although still with some evidence of the former presence of some Mg-silicates. Physical reworking of these sediments led to the dispersion of the gels and the concentration of detrital carbonate components. Despite earlier proposals, evidence of microbial processes producing carbonates in these Cretaceous lake deposits is rare and the application of facies models based on modern and ancient microbialite analogues maybe be misplaced. Carbonate reservoirs of microbial origin have been reported from the Cretaceous (Aptian) South Atlan- tic rift-sag lacustrine basins (Carminatti et al. 2008, 2009; Nakano et al. 2009) and their textures have been both illustrated from the Campos Basin, off- shore Brazil by Dias (2005) and also classified from the Barra Velha Formation, Santos Basin by Terra et al. (2010). These carbonates are often informally referred to as the ‘Microbialites’ and developed before the main phase of salt deposition (Ariri Formation; Fig. 1) in these basins, hence being also referred to as the ‘Pre-Salt Microbialites’. Terra et al. (2010) report the presence of both shrub-like (arborescent stromatolites) and spheruli- tic textures which they classify as having been bound by microbes during formation. However, to date no detailed studies have been published on these carbonates. More typical microbialite facies have been illustrated and discussed from the Lower Cretaceous of the Brazilian offshore basins. For example, Muniz & Bosence (this volume, in press) have described microbialites from the sag phase Aptian Macabu Formation of the southern Campos Basin, possibly as more marginal marine deposits. These authors developed an image log-based facies model and cycle stratigraphy for these micro- bialite carbonates. The metre-scale cyclic carbon- ates represent a transitional phase from continental to marine systems, with clastic alluvial fans close to the shoreline and more basinward carbonates deposited in shallow and restricted non-marine environments. The shallowing-up cyclothems pass from microbial laminites and bioturbated marls (deep subaqueous) through thrombolitic to bioclas- tic grainstones with ooids, to stromatolites with intermound grainstones, to microbial laminites and a subaerial exposure cap with desiccation features. The Muribeca Formation (Aptian) of the Carmo ´polis field, Sergipe–Alagoas Basin, are also sag phase carbonates and are interpreted as transi- tional marginal/lagoonal deposits by de Arau ´jo et al. (2012). The Carmo ´polis field contains microbial boundstones at the base of Ibura Member, located at a basement structural high (the Aracaju High). The depositional setting is interpreted as higher marginal areas with basement exposure and a series of shallow water ponds with occasional influx of saline waters, probably from a nearby lagoon, termed a lake – lagoon complex. Sulphates are pres- ent and the carbonates are extensively dolomitized. Lacustrine microbialite reservoirs are also known from the Barremian Toca carbonates of the offshore Congo Basin (Harris 2000), and have recently been illustrated from the Kambala Field by Wasson et al. (2012). Here the Toca consists of fossiliferous grainstones to wackestones with From:Bosence, D. W. J., Gibbons, K. A., Le Heron, D. P., Morgan, W. A., Pritchard, T. & Vining, B. A. (eds) Microbial Carbonates in Space and Time: Implications for Global Exploration and Production. Geological Society, London, Special Publications, 418, http://dx.doi.org/10.1144/SP418.3 # 2015 The Geological Society of London. For permissions: http://www.geolsoc.org.uk/permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics at University of Otago on April 23, 2015 http://sp.lyellcollection.org/ Downloaded from