Shear Versus Settling Velocity of Recent and Fossil Larger Foraminifera: New Insights on Nummulite Banks

Abstract

The genesis of nummulite banks remains a poorly understood topic and the aim of this work is to shed light on it by observing the hydrodynamic behavior of selected larger foraminifera collected from both bank and non- bank deposits. Entrainment and settling velocity of both recent and fossil larger foraminifera were measured using a flume channel and settling tube. Both velocities give information about the reaction of foraminiferal shells to different hydrodynamic conditions affecting their capability to build bank-like sedimentary structures. To assess entrainment, experiments were performed on different substrates to simulate bed load transport on smooth surfaces, fine sand, coarse sand, and bioclastic substrates. Thirty-four recent and 49 fossil shells of foraminifera were used; recent taxa used are Operculina ammonoides, Heterostegina depressa, and Palaeonummulites venosus and fossil taxa used are Nummulites perforatus, N. fabianii, and N. tavertetensis (only A forms were used in this study). Our results seem to differentiate the hydrodynamic behavior of shells collected from banks from those collected from non-bank deposits. The latter possess settling velocities significantly lower than their entrainment velocities, while for taxa collected from nummulite banks, settling velocities are always close to entrainment velocities. Therefore, the relation between hydrodynamics and shape and size may explain why modern larger foraminifera, consistently of smaller size than fossil forms, cannot produce banks and that transport as bed load in moving water was possibly the main trigger for the production of nummulite banks

Settling velocity and drag coefficient of platy shell fragments

“…The reason is that these breakdown products of organic structures vary in size, density, roundness and shape (plate, rod, disc, blade, block, sphere, etc. ), depending on the organism (for example, coral, foraminifer, mollusc or calcareous algae) and the hydrodynamic environment in which they were produced (Maiklem, ; Braithwaite, ; Kench & McLean, , ; Simões et al ., ; Briguglio et al ., ; Cuttler et al ., ).…”

Growth, chamber building rate and reproduction time of Palaeonummulites venosus (Foraminifera) under natural conditions

“…substrates, digging slightly into the sediment and thus resisting entrainment by strong hydrodynamics (Hohenegger 2004;Yordanova and Hohenegger 2007;Briguglio et al 2017). A trimorphic life cycle has not been observed , and the alternation between agamonts and gamonts is caused by hindering the evacuation of gametes after gamogony by weak hydrodynamics at water depths below 50 m (Eder et al 2017).…”

Microfacies analysis of Eocene Ziarat Formation (eastern Alborz zone, NE Iran) and paleoenvironmental implications

“…In this MFT, the formatting of these accumulations is influenced by the process of winnowing and in situ reworking where the wave and current action plays an important role accompanied by: (1) the moderate to high degrees of abrasion/ damaged (BTI:1-3) (2) observation of 'chaotic stacking' and locally 'sub-linear accumulation' and (3) the dominance of A-form Nummulites (ratio of A-to B-forms varies from 20:1 to 30:1) that is clearly in accordance with nummulite banks sensu discussed by Sarkar (2018). Briguglio et al (2017) have recently pointed out the hydrodynamic energy as a major factor in the genesis of nummulite banks. They also mentioned that other factors such as reproduction strategies, sedimentation rate, and shell growth rates must be considered to develop a comprehensive model to explain the production of such accumulations.…”