Sedimentology, diagenesis and ichnology of Cretaceous and Palaeogene calcretes and palustrine carbonates from Uruguay



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Alonso-Zarza, Ana María and Genise, Jorge F. and Verde Cataldo, Mariano (2011) Sedimentology, diagenesis and ichnology of Cretaceous and Palaeogene calcretes and palustrine carbonates from Uruguay. Sedimentary Geology, 236 . pp. 45-61. ISSN 0037-0738

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The Cretaceous (Mercedes Formation) and Paleogene (Queguay Formation) deposits cropping out inWand S
Uruguay comprise two terrestrial limestone units that are very rich in trace fossils. The study of these units
permits to propose a sedimentological model for palustrine limestones and calcretes in which the distribution
of different types of trace fossils is considered. The study units include three main types of deposit: lacustrine
limestones, palustrine limestones and calcretes. The lacustrine limestones are relatively homogeneous and
contain gastropods, charophytes and ostracods, but no trace fossils. They were deposited in a relatively more
perennial lacustrine environment. The palustrine limestones include four different facies: desiccated
mudstones, nodular limestones, granular limestones and gravel-sheets. The desiccated mudstones indicate
a lesser degree of pedogenic modification and the granular limestones a higher degree. The gravel-sheets are
an indication of the reworking of previous limestones deposits during low lakewater levels. Most of the
palustrine limestones (except the gravel sheets) contain the same bioclasts as the lacustrine limestones plus a
variety of trace fossils such as Rebuffoichnus sciuttoi, Fictovichnus gobiensis and different ichnospecies of
Celliforma. The calcretes are either massive (groundwater) or laminar. The massive calcretes are sandy
limestones made up of a carbonate matrix and cements. The laminar calcretes (root mats), which contain
alveolar septal structures, occur as centimetre-thick layers and can be seen in all types of deposit. They contain
the same trace fossil association as the palustrine limestones; the massive calcretes are poorer in such fossils.
The distribution of trace fossils in these environments is under strong facies control and provides good
evidence of subaerial exposure and semi-arid climates. All the limestones are partially replaced and cemented
by opal and quartz, but in all cases the primary structure is preserved. Silicification occurred under
groundwater meteoric conditions. Overall, the limestones facies (calcretes, palustrine and lacustrine) and
their lateral distribution likely reflect the existence of wetland environments in semiarid to sub-humid
climates. These climatic conditions were interrupted by a period of increased precipitations, probably the
Early Eocene Climatic Optimum.

Item Type:Article
Uncontrolled Keywords:Calcretes, Lacustrine carbonates, Trace fossils, Silicification, Chaco–Paranense Basin, Uruguay
Subjects:Sciences > Geology > Paleontology
Sciences > Geology > Petrology
ID Code:25332
Deposited On:08 May 2014 12:39
Last Modified:08 May 2014 12:39

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