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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
STUDENT POSTER SESSION COMPETITION
Abstract: Diagenetic Framework for Chemical
Remanence Acquisition in Lower Paleozoic
Carbonate Rocks From W. Newfoundland
By
The Lower Ordovician (Tremadocian) St. George
Group on Port au Port Peninsula forms part of the Cambro-Ordovician autochthonous carbonate sequences of W.
Newfoundland. The geology and stratigraphy of the St.
George Group indicates that these rocks have been subjected
to distinct uplift and exposure events and their petrographic characteristics suggest a complex diagenetic
history. The fabrics of these rocks range from those of
relatively pristine limestones to diagenetic dolomites and
dedolomites with only relict depositional textures. Paleomagnetic
and rock magnetic
data show that there are two
generations of hematite that record separate magnetization
events in these strata, and also indicate the presence of
magnetite. The hematite is shown to be diagenetic,
occurring in association with karst fabrics and dedolomitization
and as an alteration product of pre-existing sulfide
minerals. Three components of remanence are observed in
these rocks: 1) a recent overprint, 2) a Late Paleozoic
remagnetization, and 3) an Ordovician (but not depositional)
magnetization. The recent component is best explained as a
viscous overprint. The Late Paleozoic component is contained
in hematite, while the Early Paleozoic (Ordovician)
component is found to be contained in both magnetite and
hematite.
Paleomagnetic, petrographic, geochemical, and geologic
evidence will be presented that indicates both Paleozoic
components record chemical remanent magnetizations
associated with subaerial exposures of the ancient continental
shelf during at least two distinct times in the Paleozoic.
Given the observed range of petrographic and magnetic
variability, it appears that during the (Early - Middle ?)
Ordovician, the lower St. George rocks underwent relatively
rapid diagenesis including lithification, calcite cementation,
sulfide precipitation, dolomitization, dedolomitization, and
hematite authigenesis. The Late Paleozoic component
appears to represent a remagnetization component also
associated with dedolomitization and hematite authigenesis
and cementation. Perhaps more importantly, rock samples
bearing hematite as the major remanence phase can be
petrographically identical to one another, yet carry distinct
directional components acquired at different times in geologic
history.
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