significance Pelletal black shale fabrics: their origin and

Abstract: 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 palaeo- oxygen 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). Palaeo- ecological 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 in- fauna at time of formation (e.g. Byers 1977; Dow 1978; Pratt 1984; Savrda