GRAINS: Skeletal Fragments: Problematica
2003. "GRAINS: Skeletal Fragments: Problematica", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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There are thousands of problematic organisms — organisms unassigned to a specific phyletic group, or ones that were assigned to different groups by different workers. We have simply picked a few that are particularly distinctive and/or that are important in rocks of hydrocarbon exploration interest. We list prior phyletic assignments and age ranges below and provide descriptions and keys to recognition in the figure captions.
Receptaculitids - grouped with sponges, corals, dasycladacean green algae, or problematica — common from Early Ordovician to Late Devonian, with smaller, more globular forms extending into the Permian
Nuia - grouped with problematic codiacean algae or as an unassigned organism — Late Cambrian-Ordovician
Palaeoaplysina - grouped with sponges, phylloid algae, or hydrozoans — Mid. Pennsylvanian-Early Permian
Tubiphytes - variously grouped with cyanobacteria/blue-green algae, red algae, calcareous sponges, foraminifers, hydrozoans — at least Late Carboniferous to Late Jurassic
Lithocodium - grouped as codiacean algae or loftusiid foraminifers — Late Triassic to Early Cretaceous (Albian)
Hensonella - grouped as mollusks (scaphopods), coralline red algae, or dasycladacean green algae — Cretaceous (Hauterivian-Albian)
A cross section of the wall structure of Calathium sp., with its central cavity and moderately well-preserved radiating wall structure. Calathids are the earliest receptaculitids — they had ovoid or tubular skeletons that strongly resemble sponges (one of the groups in which receptaculitids commonly are classed). The sparry calcite-filled areas (and micrite-filled circles) are recrystallized, originally aragonitic, elongate pillars that constituted the skeletal wall (see Nitecki et al., 1999); the rest of the micritic sediment has filled areas of former void spaces or sites of later-decomposed organic tissues.
Figures & Tables
A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis
This volume expands and improves the AAPG 1978 classic, A Color Illustrated Guide to Carbonate Rock Constituents, Textures, Cements, and Porosities(AAPG Memoir 27). Carbonate petrography can be quite complicated. Changing assemblages of organisms through time, coupled with the randomness of thin-section cuts through complex shell forms, add to the difficulty of identifying skeletal grains. Furthermore, because many primary carbonate grains are composed of unstable minerals (especially aragonite and high-Mg calcite), diagenetic alteration commonly is quite extensive in carbonate rocks. The variability of inorganic and biogenic carbonate mineralogy through time, however, complicates prediction of patterns of diagenetic alteration. This book is designed to help deal with such challenges. It includes a wide variety of examples of commonly encountered skeletal and nonskeletal grains, cements, fabrics, and porosity types. It includes extensive new tables of age distributions, mineralogy, morphologic characteristics, environmental implications and keys to grain identification. It also encompasses a number of noncarbonate grains, that occur as accessory minerals in carbonate rocks or that may provide important biostratigraphic or paleoenvironmental information in carbonate strata. With this guide, students and other workers with little formal petrographic training should be able to examine thin sections or acetate peels under the microscope and interpret the main rock constituents and their depositional and diagenetic history.
- carbonate rocks
- color imagery
- problematic fossils
- sedimentary rocks