SHEAR VERSUS SETTLING VELOCITY OF RECENT AND FOSSIL LARGER FORAMINIFERA: NEW INSIGHTS ON NUMMULITE BANKS

CESARE ANDREA PAPAZZONI,MONA SEDDIGHI,JOHANN HOHENEGGER,ANTONINO BRIGUGLIO

doi:10.2110/palo.2016.083

Abstract

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)...

Highlights

Accumulations of larger foraminiferal (LF) shells are very common in sediments deposited in shallow tropical seas during the Phanerozoic (e.g., Racey 2001 and references therein)
The fossil taxa used in this study comprise 29 A-forms of Nummulites perforatus (10 collected from Capus and 19 from Leghıa, both from the Transylvanian Basin, Romania; Bartonian), 10 A-forms of N. fabianii and 10 A-forms of N. tavertetensis collected from Tavertet
The experiments conducted in this study suggest that shells possessing a critical shear velocity lower than settling velocity are more prone to be transported as bed load, whereas shells with settling velocity lower than the critical shear velocity tend to be transported as suspended load

Summary

Introduction

Accumulations of larger foraminiferal (LF) shells are very common in sediments deposited in shallow tropical seas during the Phanerozoic (e.g., Racey 2001 and references therein). Some of these accumulations, known as nummulite banks, have drawn attention for a long time as they have peculiar characteristics that are difficult to explain and their depositional environment is still a matter of debate. First defined by Arni (1965), these sedimentary bodies are characterized by extraordinary abundance of LF shells of very low diversity. They appear to be mostly composed by large microspheric agamonts (i.e., the B forms). A possible explanation in the case of LF was given by Hottinger (1999), who described the occurrence as an ‘‘odd partnership’’ in which the two taxa are characterized by striking size difference and perhaps different growth rates and reproduction intervals, allowing both to profit from the same ecological niche

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