UNIVERSITY OF CALGARY SPRING 2010

GLGY 209-20 INTRODUCTION TO GEOLOGY I

Oxic/anoxic conditions

Oxic conditions: benthic organisms at the bottom of the ocean destroy the dead organisms: low chances to fossilize.

Anoxic conditions: no benthic organisms at the bottom of the sedimentary basin: high chances to fossilize.

Example of burrowing process

Bivalve burrowers in a coral colony (recent faunas of the Bahamas).

Example of burrowing process

Carbonate mud sediments at a water depth of about 5 meters; note the presence of burrows of the crustacean Callianassa; Cat Bay, Bahamas.

Carbonate mud stabilized sediments at a water depth of about 1.5 meters; Cockroach Island, Bahamas.

Exceptional preservation

•Cambrian sediments of British Columbia (western Canada)

• Burgess Shale

Lagerstatten – Crato Formation of Brazil (approx 110 m.y.)

Insect wings (left) and a roach wing (right). The right image is acid-prepared.

Lagerstatten – Crato Formation of Brazil (approx 110 m.y.)

Various scorpion species (Protoischnurus and Araripescorpius).

Lagerstatten – Crato Formation of Brazil (approx 110 m.y.)

Fossil insect showing preserved color pattern on the wings – Baecossus.

Kinds of fossils

Body fossils: direct or altered remains of ancient organisms (e.g., teeth, shells, tests, etc).

Trace fossils (ichnofossils): evidence of the activity of ancient organisms.

Chemical fossils (biomarkers): organic compounds produced by various organisms, which are found in strata.

Examples of body fossils

Left: crinoid with the calcareous plates in anatomical connection and corals.

Right: cluster of brachiopods.

Example of trace fossil

Example of a trace fossil of a trilobite; the trilobite body is not preserved.

Examples of trace fossils

Flying traces: multiple looping trails generated by a flying insect caught on surface with a thin film of mud or water.

Beetle traces: moving traces of an insect (Scoyenia).

Examples of trace fossils

Left: root casts (rhizoliths) in an ancient

soil (Spain).

Right: reconstruction of a modern crayfish burrow (height = 0.7 m).

Kinds of fossil preservation Recrystallization; Moldic preservation of mold-cast

preservation; Perminerallization; Carbonization; Congelation; Fossilization in amber.

Recrystallization

This process involves the conversion of the original minerals of the test into a different mineral.

The most common transformation during the process of recrystallization is that from aragonite to calcite.

Aragonite and calcite are two carbonates (CaCO3) but with different crystal structures.

Moldic preservation

The original test or shell is dissolves; example of a gastropod; the shape of the former organism is given by the lithified sediment infill.

The mold (right) and cast (left) of a trilobite; note that the original material was dissolved.

MP - ReplacementReplacement is rather a particular case of moldic (mold and cast) preservation, in which the space resulted from the dissolution of the original test is filled by a new mineral.

Pyritization is the most frequent (and spectacular) case of replacement.

PermineralizationPermineralization results from the filling of the pore spaces usually by opal or chalcedony (SiO2).

Carbonization

A kind of fossilization often met in plants and algae, but also in fishes.

Occurs when the fossils are buried deep into the Earth’s crust and all the organic material is expelled, excepting for the carbon.

These fossils have a blackish appearance.

CongelationThis kind of fossilization occurs at high latitudes, where there is a thick layer of frozen soil and rock at the uppermost part of the Earth’s crust.

Fossilization in amber

A salamander, a frog and a snail fossilized in amber. Amber is a natural resin, which allow three dimensional fossilization of the organisms caught while it is still liquid (high viscosity).

Tar PitsTar pit in California from which were recovered excellently preserved fossils that lived in this area during the past 40,000 years.