Cool-Water
Carbonates: an Evolving Paradigm
James, Noel P.1 (1)
Queen'sUniversity,
The cool-water carbonate paradigm, which
states that significant parts of the neritic carbonate record accumulated in
cool ocean waters outside the tropics, is now part of mainstream scientific
thinking. Because such sediments are composed only of marine organisms, all of
whom respond in different ways to varying environmental controls, the precise
characterization of resultant sediments in the modern ocean is proving
difficult and controversial, whereas their identification in the rock record is
variably speculative and puzzling.
Such sediments belong to the heterozoan
(foramol) assemblage that is distinct from the photozoan (chlorozoan,
chloralagal) assemblage characterized by warm-water (>20°C),
light-dependent, and non-biogenic components. In the modern ocean such simple
assignments are perturbed by water depth, nature of the photic zone, changing
salinities, and varying trophic resources. This is particularly apparent in the
transition zone between temperate and tropical realms. Alternatively,
carbonates in forbidden polar environments are present, but virtually
undocumented.
Whereas Cenozoic and Mesozoic carbonate
rocks can be interpreted with confidence based on modern deposits, the
consignment of older rocks to cool- or warm-water environments is elusive. Late
Paleozoic rocks associated with glacial-marine neritic environments as opposed
to those with large benthic foraminifers (fusilinids) can be resolved. In
contrast, early and middle Paleozoic carbonates are assigned to the cool-water
realm based on the absence of warm-water elements and inclusion of prolific
biosiliceous components. The Proterozoic carbonate realm remains largely
enigmatic, new research suggests that ooids may be part of the cool-water
depositional realm on the early earth.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California