Geology Reference
In-Depth Information
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Further reading:
K182, K194
the tropical shallow-water, the cool-water and the mud-
mound production systems differ in composition, ge-
ometry and facies patterns, and show differences in
spatial distribution and occurrence in time (Sect.
16.7.2). The prevailing mineralogic composition of
non-skeletal and skeletal grains, and of micrite and ce-
ment has changed over time (Sect. 16.7.3). Temporal
differences in the mode, abundance and distribution of
some microfacies criteria must be considered using
these criteria to interpret paleoenvironmental conditions
and depositional patterns (Sect. 16.7.4).
16.7.1 Changes in Major Carbonate Deposi-
tional Environments over Time
Precambrian carbonates.
The rates of carbonate sedi-
mentation varied through time, starting as early as the
Archean (Hay 1999; Grotzinger and James 2000). The
temporal progression of major carbonate facies types
during the Precambrian provides a record of long-term
chemical evolution of sea water and of the biological
evolution. Archean and Paleoproterozoic carbonates are
characterized by abundant sea floor precipitates. Neo-
proterozoic carbonates were dominated by clastic-tex-
tured facies and carbonate mudstones. Mesoproterozoic
and Early Neoproterozoic carbonates contain the enig-
matic 'molar-tooth' structures. Many Neoproterozoic
grainstones are unique because of the occurrence of
giant, centimeter-sized calcareous ooids.
Reefs appeared early in the Archean. Archean to
Mesoproterozoic reefs are predominantly stromatolite-
based. Lamination textures indicate a shift from in-situ
precipitation of aragonite and calcite encrusting the sea
floor to textures formed by accretion of loose sediment
through trapping and binding in Neoproterozoic stro-
matolites. This trend reflects a decrease in abiotic fac-
tors and an increase in the control of stromatolites by
benthic microbial mats. Terminal thrombolitic Protero-
zoic reefs contain the first calcified metazoans.
Snowball Earth.
The currently debated Snowball
Earth hypothesis postulates a global glaciation during
Early and Late Proterozoic time. The Neoproterozoic
glacigene sediments are covered worldwide by a car-
bonate unit, typically dolostone. Some of the best pre-
served cap carbonates are developed on 'Marinon'
glacigene sediments in the Mackenzie Mountains,
Canada (James 2001). It consists of two units, a lower
unit interpreted as a worldwide precipitation event, with
laminated, peloidal and stromatolitic dolostones con-
taining conspicuous tepees, and an upper transgressive
unit with peloidal grainstones to cementstones, grad-
ing upward into thick shale. These carbonates are char-
16.7 Secular Variations in Carbonate
Depositional Patterns and Temporal
Changes in Microfacies Criteria
Carbonate depositional systems as well as microfacies
criteria exhibited secular variations during the Phan-
erozoic. Changes and overall trend patterns are reflected
by the composition and distribution of platform car-
bonates, reef limestones and pelagic sediments (Sect.
16.7.1). Marine benthic carbonate factories, including
