R. M. Stainforth
The Micropaleontologist 1952 - 6:1:42
There are numerous scattered references in the literature to foraminiferal assemblages composed almost exclusively of arenaceous species. These faunas may be split readily into two groups, the first of which consists of tiny arenaceous foraminifera with flimsy tests. Faunules of this group are known only from brackish-water facies such as parts of the Mississippi and Orinoco deltas and a salt marsh in Trinidad (see Lowman, 1949, pp. 1950-1963; Cushman and Brönnimann, 1948).
The second group consists of faunas strongly dominated by large, robust, arenaceous foraminifera. Families well represented are the Rhizamminidae, Reophacidae, Ammodiscidae, Lituolidae, Textulariidae, Verneuilinidae and Valvulinidae. Such references as have been made to the ecology of these assemblages tend to imply deep and/or cold marine conditions (see Glaessner, 1945, pp. 189-190; Galloway, 1933, pp. 58, 95, 173, 193). This opinion probably rests partly on Recent samples from abyssal depths at which the tests of calcareous organisms are dissolved (see Cushman, 1948, p. 44), and partly on records of individual species in dominantly calcareous assemblages.
Some years ago the problem of the ecology of this type of fauna arose in Trinidad. In facies studies made under the supervision of Dr. H. H. Renz, we found that these faunas were typical of three distinct facies, namely (1) the Tarouba, Chaudiere and Nariva formations, a piedmont facies (flysch) produced by rapid sedimentation resulting from active erosion of a rising land mass: (2) colloidal clays interbedded in the deltaic sediments of the Cruse formation; (3) certain clays in the Ste. Croix beds, believed to represent the redeposition, in rather deep water, of submarine mud extrusions (see Renz, 1942, pp. 528-529, 547, 559; Stainforth, 1948, pp. 1308, 1317). We concluded that neither depth, temperature nor salinity could be a major factor in the development of the arenaceous microfaunas. Our tentative conclusion was that turbidity was the controlling ecologic factor. The reasoning behind this conclusion was twofold: First, turbidity is the most obvious factor common to the three facies studied but unimportant in normal marine facies where arenaceous and calcareous foraminifera coexist; second, photosynthesis is vital to the existence of calcareous organisms and is reduced or eliminated in areas of highly turbid water.
Records from other countries appear to support the concept that wholly arenaceous microfaunas represent turbid-water environments. Cases known to the writer are:
Venezuela: The Capiricual member of the Santa Inez formation (see Hedberg and Pyre, 1944, pp. 20-24 ). Environment: A flysch facies, closely analogous to the Nariva formation of Trinidad.
Peru: The Mal Paso formation (see Frizzell, 1943). Environment: A “dump deposit” formed by products of rapid erosion of a rising fault scarp.
Ecuador: The Estancia formation (see Thalmann, 1946, p. 346). Environment: Probably analogous to the Mal Paso formation of Peru.
Austria: The Alpine Flysch (see Grill, 1950, pp. 51, 59);
Italy: The Oligocene Flysch (see Lipparini, 1951, pp. 5-6). Environment: The Alpine Flysch is regarded as a large-scale “dump deposit” or piedmont facies consisting of the products of erosion from the rising Alpine chain. The name is used, as above, as a general term for any similar facies (see Gignoux, 1950, pp. 567-572).
Poland: Czarnorzeki beds, etc., of the Carpathian Flysch (see Pozaryski, fide Bieda, 1950). Environment: Presumably like the Alpine Flysch.
Japan: Oil source-rocks in the oil fields (fide Asano, personal communication). Environment: Possibly a flysch facies in the “Sagara geosyncline” (see Hanzawa, 1950, pp. 81-85).
I have not encountered any satisfactory records of Recent faunas which resemble the fossil examples cited. Dr. K. Asano kindly informed me that one-genus assemblages of Cyclammina spp. have been dredged from the seas surrounding the Japanese islands, but some special ecologic control must be suspected in this case. It would be of general interest if someone could produce Recent material with which to check the idea that turbidity is a prerequisite for the development of arenaceous microfaunas of the type under discussion.