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Seminar - Joao Trabucho: "Black shales: global forcing mechanisms or local sedimentary processes?"

'Black shale' has been historically used to refer to a general class of dark, organic matter-rich, fine-grained marine sedimentary rocks. Although we use one collective noun to refer to these rocks, they are very varied in petrological terms and they are the sedimentary products of a variety of processes, or combination thereof, in many different depositional environments. The distribution of black shales in the Phanerozoic sedimentary record is remarkably uneven. Even though fine-grained sedimentary rocks are the most ubiquitous component of the sedimentary record, black shales are mainly limited to six stratigraphical intervals of the Phanerozoic, which account for one-third of Phanerozoic time. These intervals are characterised by significant perturbations in the global carbon cycle, expressed in the rock record as excursions in carbon isotopes, and, particularly in the Mesozoic, they are referred to as oceanic anoxic events (OAEs). The geographical distribution of black shales during an OAE is 'widespread'; however, whilst black shales are typically regarded as the lithological expression of OAEs, the carbon isotope excursions that define these intervals are often found in organic matter-lean sediments. Indeed, the concept of OAE implies neither global anoxia nor synchronous global deposition of organic matter-rich lithologies, but rather an interbasinal propensity for the accumulation and preservation of organic matter in sediments. Despite its undisputed convenience, an unfortunate side effect of the usage of 'black shale' to refer to organic matter-rich marine sediments is that it leads to a nonprocess-based correlation of very different facies (based on carbon isotope records), i.e. they are interpreted to be sedimentary products of the same mechanisms. Moreover, the interpretation of environments of black shale deposition is dominated by three oversimplified uniformitarian models (viz. restricted circulation, open ocean oxygen minimum and continental shelf models) which do not capture the complexity and dynamics of environments of black shale deposition. Correlation of coeval 'black shales' without taking into account their sedimentological characteristics and the local processes that led to their formation is likely to lead to unsound palaeoenvironmental interpretations. The fact that black shales are coeval does not mean that similar processes produced them, or that depositional environments were similar; rather, each black shale should be interpreted individually after which a meaningful correlation in terms of processes may be achieved. The sedimentological study of black shales shows that many of the assumptions held as valid in the current OAE paradigm are false. In this seminar, I will show how a process-based correlation of black shales leads to a better understanding of their geographical distribution. Global forcing mechanisms played a role in the formation of organic matter-rich sediments. In particular, they controlled the uneven distribution of black shales in the Phanerozoic. However, local processes ultimately controlled which sediments were deposited at a specific location, and whether they were organic matter-rich or not.
When Dec 05, 2011
from 02:00 to 03:00
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