GRAINS: Skeletal Fragments: Sponges and Related Groups
2003. "GRAINS: Skeletal Fragments: Sponges and Related Groups", A Color Guide to the Petrography of Carbonate Rocks: Grains, textures, porosity, diagenesis, Peter A. Scholle, Dana S. Ulmer-Scholle
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This short-lived but widespread group have been classed by various as sponges, corals, or calcareous algae. Archaeocyaths now are almost unversally considered as subphylum of the Porifera (possibly related to the demosponges); a few workers still group them in a separate phylum (Phylum Archaeocyatha). They range mainly from basal Cambrian to late Early Cambrian (a few forms persisted to Middle and Late Cambrian).
One of the earliest groups to secrete substantial skeletal calcium carbonate and the first reef-building organism.
Sessile, benthic, filter feeders. Exclusively marine organisms that lived in tropical, normal salinity (ca. 30-40 ppt) waters at depths from the intertidal zone to a few tens of meters, mainly in areas with relatively low influx of terrigenous sediments (see Debrenne and Reitner, 2001).
Constructed small bioherms in association with calcimicrobes. They also are found in lesser abundance, size and diversity in inter-bioherm areas.
The good preservation of most archaeocyath skeletal material indicates a primary calcitic composition.
Most archaeocyaths have a solitary cup- or bowl-shaped skeleton that has a pair of porous walls enclosing a large central cavity. The inner and outer walls have a series of spherical perforations and are connected by numerous perforate or imperforate partitions (vertical septa and horizontal tabulae).
Less commonly, archaeocyaths had branched, massive, or chain-like colonial forms.
The average size of archaeocyath cups is 1 to 2.5 cm in diameter and 15 cm in height. Cups as small as 2-3 mm or as large as 60 cm in diameter are known, however.
Closely associated with calcimicrobial encrusters such as Renalcis and/or Epiphyton, and external morphology is commonly outlined by such encrusters.
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