Anatomy of a Recent Biodetrital Mud-Mound, Florida Bay, USA

  1. C. L. V. Monty,
  2. D. W. J. Bosence,
  3. P. H. Bridges and
  4. B. R. Pratt
  1. D. W. J. Bosence

Published Online: 14 APR 2009

DOI: 10.1002/9781444304114.ch17

Carbonate Mud-Mounds: Their Origin and Evolution

Carbonate Mud-Mounds: Their Origin and Evolution

How to Cite

Bosence, D. W. J. (2009) Anatomy of a Recent Biodetrital Mud-Mound, Florida Bay, USA, in Carbonate Mud-Mounds: Their Origin and Evolution (eds C. L. V. Monty, D. W. J. Bosence, P. H. Bridges and B. R. Pratt), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304114.ch17

Author Information

  1. Department of Geology, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

Publication History

  1. Published Online: 14 APR 2009
  2. Published Print: 17 JUL 1995

ISBN Information

Print ISBN: 9780865429338

Online ISBN: 9781444304114

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Keywords:

  • Biodetrital mounds differ from microbial mounds;
  • X-ray diffraction (XRD) analysis for carbonate mineralogy;
  • Basal packstone;
  • bioturbated peloidal muds with little grain orientation and no lamination;
  • mud ‘whitings’;
  • mounds-from storm currents with high suspended and bed-load

Summary

Initial investigations of the Recent mud-mounds of Florida Bay suggested that they are biogenic structures formed from the trapping and binding of locally produced (mainly algal) aragonitic mud. Later work showed that the mound sediments are composed of both calcite and aragonite derived from the breakdown of a range of shelly biotas within the bay. Sediment generated within the bay is transported, as both suspended- and bed-load, largely by NE wind-driven currents, towards the Gulf of Mexico to the SW. Recently the mounds in the centre of the bay have been shown to have low production rates, SW progradational geometries and physical rather than biogenic depositional structures. Large mounds at the mouth of the Bay have faster production rates, both progradational and aggradational sequences, and grow from amalgamation of former SW prograding mounds.

A detailed study of Upper Cross Bank, based on 67 cores, illustrates the sedimentary geometries, facies and evolution of central Florida Bay mounds. Measurements on staked margins to the mounds indicate decimetre-scale erosion and deposition rates over a 5-year period. Prograding and aggrading lenses of the following facies are found: basal (mollusc, intraclast) packstones; layered (mollusc, foraminifera) mudstone; skeletal (mollusc, foraminifera) packstone; winnowed, skeletal (foraminifera, mollusc) grainstone; and grass-bed (peloid, mollusc, foraminifera) wackestone. Pelleted muds form 10% of windward and 40% of leeward surface facies. Muds are predominantly silt-sized (4–8ϕ) grains composed of aragonite and high-Mg calcite. SEM examination of muds of this modal size indicates skeletal fragments together with grains of plate, bun and prismatic shapes consistent with a mixed skeletal origin.

Strontium versus aragonite values indicate that the mud is of one population generated through local breakdown of low-aragonite, low-strontium foraminifera, Thalassia encrusters and lithoclasts, and high-aragonite, high-strontium green algae.

The stratigraphy of these mounds indicates that the earliest evidence of a mound comes from aggradational grass-bed wackestones with tin wedges of layered mudstones. These are overlain with erosional contact by NE-derived storm-beds and then SE-derived layered mudstones. Present-day sedimentation is controlled by NE erosion with winnowed grainstones and mud-pebble conglomerates and SW (leeward) deposition in grass-beds and mud-dunes. Thus the facies anatomy within the mound is a complex response of storm-weather physical deposition and fairweather biogenic and physical deposition.

Biodetrital mounds differ from microbial mounds in their setting, sediment geometries, facies, structures, allochems, and mud size and composition.