Pelletal black shale fabrics: their origin and significance

Recent work has demonstrated that, under certain conditions, laminated mudstones may be produced from the compaction of highly pelletized, organic-rich sediments occupied by populations of marine benthic worms. Such sediments can be found at the most developed part of the dysoxic region in many modern minimum zones. In order to reconstruct this facies accurately in the rock record, it is necessary to discern the true origin of pellets (benthic or planktonic) present in black, laminated shales. Petrographic techniques, electron microscopy and microprobe analysis were employed on modern benthic and planktonic faecal pellets in order to arrive at inorganic geochemical criteria useful for discriminating between them. Benthic pellets contained more of those elements found in the fine fraction of the sediments (A1, Mg, K) while planktonic pellets contained high amounts of those elements (Si) representing the inorganic remains of the food they consume. These criteria were successfully applied to pellets and their enclosing sedimentary matrices in ancient laminated black shales. Systematic identification of the origins of pellets found in laminated black shales should allow for detailed identification of palaeooxygen gradients within ancient depositional environments and may reveal that truly anoxic and azoic marine conditions were much less widespread than is commonly thought. Extensive deposits of black laminated shales exist throughout the Palaeozoic and Mesozoic geological record and have been interpreted as having been deposited under totally anoxic, sometimes sulphidic, conditions (e.g. Fischer & Arthur 1977; Berry & Wilde 1978; Demaison & Moore 1980; Parrish 1982; Pratt 1984). Palaeoecological models constructed to explain the development of laminations in these shales have generally taken the presence of laminations to indicate an absence of benthic bioturbating infauna at time of formation (e.g. Byers 1977; Dow 1978; Pratt 1984; Savrda et al. 1984; Savrda & Bottjer 1987a). Recent work on modern sediments underlying anoxic and dysoxic marine waters and on the preservability of biological traces present in such sediments, however, has called this assumption into question. If dark laminated mudstones can be formed under both dysoxic and anoxic conditions, how can accurate and detailed palaeoecological reconstructions of ancient oxygen minimum zones (OMZ) and other dysoxic and anoxic marine environments be constructed? It is the purpose of this paper to explore this question in detail and to describe a potential palaeoecological/sedimentological method for resolving whether sediments were 1 Present address: Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. deposited under anoxic or dysoxic conditions. Rhoads & Morse (1971) proposed an ecological model relating bottom water oxygen conditions to the presence or absence of benthic hard parts and traces in sediments. Their hypothesis, based on modern observations of OMZs suggested that dissolved oxygen (D.O.) levels below 0.3 ml/1 would be restrictive to most macrofaunal organisms, especially those belonging to calcified taxa. They concluded that sediments underlying regions where the D.O. levels ranged between 0.3 and 0.1 roll1 would display an increasing degree of lamination corresponding to a decrease in the presence of bioturbating infauna. Byers (1977) applied the R h o a d s M o r s e model to the rock record in his reconstruction of a portion of the Upper Devonian Catskill Shelf and Basin complex in New York State. He formulated a bioturbation index which related the degree of preserved bioturbational structures to the D.O. content present at the time of deposition. Preserved fabrics observed by Byers were characterized as either fully homogenized, burrow mottled, broken and disturbed laminations or fully laminated. Additionally, the presence or absence of body fossils was noted. Studies of the California borderland basins conducted by various workers (e.g. Edwards 1985; Mullins et al. 1985; Thompson et al. 1985) have shown that, contrary to the R h o a d s From TYSON, R. V. & PEARSON, T. H. (eds), 1991, Modern and Ancient Continental ShelfAnoxia Geological Society Special Publication No 58, pp 221-232. 221 at Yale University on August 2, 2014 http://sp.lyellcollection.org/ Downloaded from