Paleontological Guides to Depositional Environments: ABSTRACT

Abstract

Foraminiferal-environmental relations provide criteria for (1) the reconstruction of marine basins of the geologic past, (2) the determination of structural trends based on differential subsidence rates, and (3) the identification of important producing trends within the sediment and environmental framework. One of the primary guides to depositional environments is the patent foraminiferal zonation from marsh to deep-sea environments. This may be summarized for southern California as follows: (1) marsh, Jadammina-Miliammina fauna; (2) euryhaline lagoon, Ammonia beccarii tepida fauna; (3) intertidal zone, Rotorbinella fauna; (4) open ocean 0-20 m., Buliminella elegantissima type of fauna; (5) 20-100 m., Florilus-Nonionella fauna; (6) 100 m. upper depth limit, Bolivina acuminata-Uvigerina peregrina fauna; (7) 400 m. upper depth limit, Bolivina argentea-Bolivina spissa fauna; (8) 700 m. upper depth limit, Bulimina striata mexicana fauna; (9) 1,000 m. upper depth limit, Uvigerina hispida fauna; and (10) 2,400 m. upper depth limit, Melonis pompilioides-Uvigerina senticosa fauna. Bathyal specie such as Uvigerina peregrina, hispida, senticosa, and Melonis pompilioides are essentially isobathyal showing little if any evidence of temperature control in their distribution patterns in different oceanic areas. Other guides to depositional environments include the bathyal bolivine trend within the general oxygen minimum zone (0.3-0.7 ml./l.); specimens of this group become larger and more abundant with increasing organic content and depth. Oxygen values of about 0.1 ml./l. or less result in an absence of larger invertebrates and a concentration of depositional laminae with very fragile hyaline bolivines and other Foraminifera. The largest benthic bathyal Foraminifera (measuring several millimeters in length) require oxygen values of more than 1.5 ml./l. and nitrogen values of more than 0.15 per cent; these occur in homogeneous or disturbed sediments, characteristics resulting from the activities of larger invertebrates. Displacement processes may produce intercalations of these various facie ; variations in water masses may result in somewhat similar fluctuations of facies. Planktonic foraminiferal abundance and diversity increase seaward into the bathyal zone. Although most planktonic Foraminifera live in the photic zone, some such as Globigerina pachyderma (typical form) and Sphaeroidinella are characteristic of the bathyal zone when they are fully developed. Many planktonic Foraminifera add a crystalline crust to their test as they descend into deeper waters of the bathyal zone. Radiolarians are of primary importance in middle bathyal-to-abyssal depths. End_of_Article - Last_Page 453------------