Aptian–Albian Planktic Foraminifera from Dsdp Site 364 (Offshore Angola): Biostratigraphy, Paleoecology, and Paleoceanographic Significance
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Journal of Foraminiferal Research, v. 43, no. 4, p. 443–463, October 2013 APTIAN–ALBIAN PLANKTIC FORAMINIFERA FROM DSDP SITE 364 (OFFSHORE ANGOLA): BIOSTRATIGRAPHY, PALEOECOLOGY, AND PALEOCEANOGRAPHIC SIGNIFICANCE KARLOS G. D. KOCHHANN1,4,EDUARDO A. M. KOUTSOUKOS2,GERSON FAUTH1 AND ALCIDES N. SIAL3 ABSTRACT and Point (GSSP) at the base of the Albian stage, although This work presents a taxonomic, biostratigraphic and several candidates have been proposed (e.g., Kennedy and paleoecological study of planktic foraminifera recovered from others, 2000; Hancock, 2001),includingapromising the Aptian–Albian carbonate-dominated succession of Deep sequence at Pre´-Guittard (Vocontian Basin, SE France; Sea Drilling Project (DSDP) Site 364, located in the Kwanza Petrizzo and others, 2012). The main reason for this Basin (offshore Angola). Twenty-nine planktic foraminiferal indefiniteness is the elevated degree of endemism of the species were identified, enabling the identification of late ammonoid assemblages, commonly used to define Creta- Aptian–late Albian biozones, from the Hedbergella trocoidea ceous stages boundaries (Birkelund and others, 1984). This Zone to the Pseudothalmanninella ticinensis Zone. A major problem causes several discrepancies between age assign- unconformity from the latest early–earliest late Albian was ments when biostratigraphic schemes based on different identified in core 31, with the Microhedbergella rischi Zone in fossil groups are applied to the same sedimentary succession direct contact with the Pseudothalmanninella ticinensis Zone. (e.g., Bolli and others, 1978), making it difficult to study the The recovered assemblages are characterized by open marine paleoceanographic events mentioned above (e.g., OAEs). epipelagic dwellers and indicate predominant mesotrophic The objectives of this study are to present the taxonomy environmental conditions throughout the studied stratigraphic and biostratigraphy of the Aptian–Albian planktic forami- succession. Aptian planktic foraminiferal assemblages have a nifera recovered from Deep Sea Drilling Project (DSDP) tropical/subtropical paleobiogeographic affinity, supporting a Site 364 (Fig. 1), Kwanza Basin (offshore Angola), and to surface-water connection between the central proto-Atlantic use their distributions to infer paleoecological aspects of the Ocean and the northern South Atlantic Ocean (north of the studied fauna and their paleoceanographic significance. Walvis Ridge-Rio Grande Rise) by the late Aptian. Trends in Despite the generally poor preservation of the studied isotopic values for d13C suggest a late Aptian age (Globiger- material, poor core recovery and absence of some tropical/ inelloides algerianus Zone) for the stratigraphic interval from subtropical biostratigraphic markers, the studied fauna core 42 to ,core 37, where age-diagnostic foraminiferal remains relevant for a better understanding of the Aptian– species are missing. Black shale levels in cores 42–39 are Albian interval and associated environmental conditions of probably the local expression of the ‘‘late Aptian anoxic the northern South Atlantic Ocean (north of the Walvis event.’’ Ridge-Rio Grande Rise). INTRODUCTION GEOLOGICAL SETTING The Aptian–Albian is a unique time interval in Earth’s Se´ranne and Anka (2005) proposed the physiographic history due to the common occurrence of organic-rich subdivision of the African margin into the equatorial sedimentary rocks, which are frequently related to oceanic western African margin (with the occurrence of evaporitic anoxic events (OAEs; e.g., Arthur and Premoli Silva, 1982; and carbonate units in the Cretaceous) and the southwest Jenkyns, 1995; Jenkyns and Wilson, 1999). These anoxic African margin (dominated by clastic deposition in the events are characterized by the widespread occurrence of Cretaceous), which are divided by the high-standing Walvis black shales and drastic shifts in the carbon isotope ratio Ridge (Fig. 1). Historically, the entire basin and continental (Leckie and others, 2002), leading to major reorganizations shelf of the equatorial western African margin has been in the marine ecosystem, including planktic foraminifera called the Angola Basin, as it is the largest depression in the (e.g., Leckie, 1984; Koutsoukos and others, 1991b; Premoli northeastern South Atlantic Ocean. Its physical isolation by Silva and others, 1999; Leckie and others, 2002). Although the Walvis Ridge was inherited from its origin in the events in this time interval are highly relevant for Barremian–Aptian (cf. Se´ranne and Anka, 2005), allowing understanding coupled global paleoceanographic and the formation of a thick layer of evaporites that exceeds paleoclimatic changes, stratigraphic problems may hamper 3 km in some locations. In the context of the Angola Basin, their accurate investigation. For instance, no section has recent studies proposed the recognition of the Kwanza been chosen yet for a Global Boundary Stratotype Section Basin, in which DSDP Site 364 is located (cf. Brownfield 1 ITT FOSSIL—Instituto Tecnolo´gico de Micropaleontologia, Uni- and Charpentier, 2006), classified as an Atlantic-type versidade do Vale do Rio dos Sinos, Bloco 6K, Av. UNISINOS, 950, marginal sag basin with the deposition of evaporitic units 93022-000, Sa˜o Leopoldo, RS, Brazil during the Aptian (e.g., Clifford, 1986; Brownfield and 2 Institut fu¨r Geowissenschaften, Universita¨t Heidelberg, Im Neuen- Charpentier, 2006). The Kwanza Basin is bordered to the heimer Feld 234, 69120 Heidelberg, Germany north by the Ambriz Arch, to the south by the Benguela 3 3 NEG-LABISE, Departamento de Geologia, Universidade Fed- eral de Pernambuco, 50670-000, Recife, PE, Brazil High, to the east by the edge of the sedimentary basins and 4 Correspondence author. E-mail: [email protected] to the west by the 4-km bathymetric depth. 443 444 KOCHHANN AND OTHERS 2000; Azevedo, 2004; Bengtson and others, 2007; Arai, 2009). During the late Aptian–early Albian, the Kwanza Basin had stagnant bottom-water conditions that prevented oxidation of organic matter (Bolli and others, 1978). Albian marly-chalks and limestones at DSDP Site 364 contain pressure-solution stylolites, steeply dipping bedding contacts, overturned folds, and interformational breccias, probably linked to salt diapirism and/or gravitational flows due to a slope paleophysiography (Bolli and others, 1978). The sedimentary rocks recovered in the deepest cores from this site suggest outer-shelf or shelf-break depths during black shale deposition (Bolli and others, 1978). MATERIAL AND METHODS DSDP Site 364 is located offshore of Angola, southwest of Luanda (11u34.329S; 11u58.309E), at a water depth of 2448 m. The studied Aptian–Albian section comprises the interval from cores 42–24 [1033.5–672.5 mbsf (meters below the seafloor)] that were spot-cored at an average spacing of 10 m with 61% average recovery (Bolli and others, 1978). Three lithologic units (Bolli and others, 1978) occur in the studied stratigraphic interval (units 7–5 from bottom to top; Fig. 2). Unit 7 is composed of marly dolomitic limestones and black shales, divided into two subunits based on the greater proportion of black shales intervals in subunit 7b than in 7a. Unit 6 contains intercalations of limestones and mudstones, which was divided into subunits a, b, and c FIGURE 1. Paleogeographic reconstruction at 112 Ma, modified based on changes of color and induration. Unit 5 (subunit from Hay and others (1999), presenting the inferred location of DSDP 5b) is a marly chalk with black shales. Site 364, Walvis Ridge, and the Guinean Gulf. Seventy-four samples were collected for the micropale- ontological survey, one sample per section with an average Se´ranne and Anka (2005) subdivided the geological spacing of 1.5 m within cored intervals. Approximately 20 g record of the equatorial western African margin (including of each sample were crushed and soaked in a 200-ml the Kwanza Basin) into four main phases: rifting, rift–drift hydrogen peroxide solution (29% H2O2) for 24 hours. For transitional, drift, and late drift. According to these indurated limestones the acetolysis method of Rodrigues authors, the rift–drift transitional phase is characterized and others (2012, experiment 12) was used to extract by a transgresssive clastic sequence, grading upwards from foraminiferal tests. The remaining residues were washed fluvial sandstones and lagoonal shales to thick evaporites, through a 38-mm sieve, and at least 300 planktic specimens whose deposition during the mid- to late Aptian is a were identified from each sample under a stereomicroscope. distinctive feature of this phase. The marine transgression, After an ultrasonic bath, scanning electron micrographs of initiated by evaporite formation, was followed by deposi- the specimens were taken to ensure accurate identifications tion of shallow-water carbonates (dolomites and limestones using wall microstructures. Abundances of planktic fora- studied herein) during the late Aptian–Albian (Binga and miniferal species were estimated relative to the total Pinda formations of Se´ranne and Anka, 2005, and planktic assemblage. First occurrences (FOs) and last Brownfield and Charpentier, 2006; or Pinda Group of occurrences (LOs) of marker species were used to identify Valle and others, 2001; mentioned herein as units 5–7). biostratigraphic events, mainly following Petrizzo and The breakup of the African and South American plates Huber (2006), Huber and Leckie (2011), and Petrizzo and began in the Early Jurassic