EPEIRIC SEAS: A CONTINENTAL EXTENSION OF SHELF BIOTAS
1. Overview 1.1. Controls 1.2. Temporal Distribution 1.3. Geographic Distribution 1.4. Sedimentology 1.5. Climatic Impact 2. The Environment of Epeiric Seaways 3. Origin and Evolution of the Epicontinental Sea Biota 3.1. Nature of the Epicontinental Fossil Record 3.2. Measuring Biodiversity in Epicontinental Seas 3.3. The Endemism Question 3.4. Precambrian 3.5. Paleozoic 3.5.1. Cambrian 3.5.2. Ordovician–Permian 3.6. Mesoand Cenozoic 3.6.1. Triassic–Cretaceous 3.6.2. Paleocene–Recent 3.7. Mass Extinctions Glossary Bibliography Biographical Sketch
The birth and demise of the vast epicontinental Permian Irati-Whitehill sea: Evidence from organic geochemistry, geochronology, and paleogeography
Abstract Organic-rich shale deposits of the Irati (Brazil) and Whitehill (South Africa) formations represent the most important sedimentary remnants of a huge Permian sea that occupied an estimated 5 million km2 before drying out and disappearing. To unravel the evolution and final demise of the Irati-Whitehill sea, we present a paleogeographic and organic geochemical characterization as well as a precise tephrochronology of the sea. Constrained by new high-resolution chemostratigraphic data (biomarker data combined with carbon and nitrogen isotopes) of two boreholes from central and southern areas of the Parana Basin (Brazil), the evolution of the sea is divided into five stages. These stages reflect the transition from a marine realm (stage I) to an environmentally restricted sea characterized by major ecosystem changes (stages II and IV) where water renewal controlled the post-salinity environmental dynamics (stages III and V). A zircon U Pb TIMS age of 277.26 ± 0.62 Ma of stage V correlates with the age of the San-Rafaelic orogeny along the paleo-Pacific South American margin and the rising of the Cape Fold Belt. We conclude that the convergent tectonic dynamics may have acted as the main agent in the entrapment and disappearance of the sea, ending a history of no more than four-million-year of the Irati-Whitehill sea.
Bahaman Calcareous Sands
The major part of the Bahama Banks is covered with a mantle of sands composed of calcium carbonate. They rest on a basement of similar sediment which has been consolidated into limestone by the subaerial deposition of calcite cement. Along the extreme edges of the Banks, the sand consists of the debris of neritic organic skeletons; elsewhere it is predominantly non-skeletal, and composed of grains of cryptocrystalline aragonite. These non-skeletal sand grains have hitherto been regarded as a break-down product of the limestones, but the present study, based on the Ragged Island area in the southeast part of the Great Bahama Bank, disproves this, and shows that such derived grains are rare in the recent bottom sediment. The sand grains are instead considered to be primary: they have formed, and are now forming, by the physico-chemical and bio-chemical extraction of aragonite from the sea water, which is saturated or supersaturated with calcium carbonate. The grains develop by the progressive aragonitic cementation of friable aggregates of calcareous silt particles. Further cementation tends to join the grains into lumps, in which an outer dense cryptocrystalline layer commonly allows the slower growth of larger aragonite prisms in the interior parts. The habit and surface texture of the lumps undergo a sequence of changes when traced from the ocean edge toward the interior of the Banks. Corrasion prevents excessive lump growth, and the typical bottom sediment is a well sorted medium-grade sand. The traces of calcareous mud in the sands contain no evidence of "drewite" needles, which are restricted to the protected "shelf lagoons" west of Andros and Abaco Islands. The controlling factor in the formation and distribution of the sands is the tidal currents of cooler oceanic water which sweep onto the Banks with velocities diminishing toward the interior, precipitating calcium carbonate as the water warms and evaporates. Sedimentation is therefore most rapid along the borders of the Banks. Aided by sea-level changes, this has led to the building of lines of elongate cays, parallel, and close to the ocean edge, making each Bank a gigantic atoll. Superficial aragonite ooliths develop on the protected beaches of the cays, and in the track of currents through the inter-cay channels. East of the "Tongue of the Ocean" and elsewhere close to End_Page 1------------------------------ the borders of the Banks, there are extensive areas where the sand is swept into a curvilinear pattern-work of submarine dunes, which occasionally adopt barchan shape. These have not been sampled, but are considered favorable for the formation of true, as opposed to superficial, ooliths. Evidence from the well drilled to a depth of 14,587 feet on Andros Island suggests that calcareous sands, similar to the recent deposits, have played a large part in building the Bahama Banks.