Depositional and Early Diagenetic Controls on Texture and Fabric of Carbonate Mudbanks, South Florida
This chapter summarizes the processes of growth and early biological modification in modern carbonate mudbanks and shows how they define a predictable texture and fabric. This is possible because, during the past two decades, the carbonate mudbanks of south Florida have been re-examined through a series of detailed stratigraphic studies (Wanless, 1976 and 1979; Warzeski, 1976; Enos and Perkins, 1979; Cottrell, 1989; Tagett, 1989; Wanless and Tagett, 1989; Wanless et al., 1989). These studies have led to a significant revision in our understanding of the origin and growth history of these mudbanks. Mudbanks nucleate in close association with inundated Pleistocene or Holocene topographic highs and irregularities. The rate and position of sea-level rise and the physiographic/hydrographic setting define timing of initiation and the geometry of carbonate bank growth.
The carbonate mudbanks of south Florida have grown as physically dynamic sediment bodies and contain both physical and biogenic depositional facies. Mudbank facies are influenced to varying degrees by biological processes of sediment production, aggregation and substrate stabilization. Depositional facies have been partially to completely transformed by biological influences inherent to the carbonate bank environments. Four depositional facies and two early diagenetic facies are recognized. Each shows great textural variation, depending on setting.
On exposed marginal settings, depositional units include: (1) fining-upwards units (0.1 to 1 m in thickness) of layered rudstone to fine grainstone, which are the dominant bank building facies forming onbank and offbank tempestite lobes; (2) fine-grained skeletal and peloidal grainstones, generated by current baffling and sediment trapping by seagrass communities in areas not catastrophically smothered or eroded by frequent storms; (3) autochthonous coralgal rudstones on bank flanks and channel margins in areas of increased agitation associated with bank shoaling; and (4) sorted grainstones to rudstones, which represent frequent reworking associated with higher energy flanks.
On protected and interior settings: (1) layered mudstone units 0.1 to 1.2 m in thickness (with or without a grainstone base) are the dominant bank-building facies; (2) peloidal wackestones to packstones form in areas of seagrass stabilization; (3) autochthonous biogenic deposits of calcareous algal grainstones (Halimeda opuntia) and mudstones (Acetabularia) occur with increased circulation and light with shoaling; and (4) sorted grainstones to rudstones cap bank interior areas and represent frequent reworking associated with shallowing to the intertidal zone.
Early diagenetic modification of these depositional units includes (5) rooting, pelleting, and bioturbation associated with seagrass stabilization, and (6) partial to complete facies transformation by communities of deep excavating burrowers. Repetitive excavation of deep open burrow networks and storm filling of networks with mud-poor packstones can result in partial to complete destruction of depositional facies and replacive generation of new sedimentary fabrics, enhanced porosity and permeability, modified sediment composition, and a changed diagenetic and dolomitization potential. Burrow-transformed facies dominate the broad interior (core) of most banks where the surface is gradually accreting. This is a dominating influence on carbonate banks (and level-bottom platforms) influenced by more normal marine waters. Banks associated with the restricted waters of inner Florida Bay are not significantly transformed, because a deep, excavating, burrowing community is not present.
The fabric, porosity, and diagenetic potential of carbonate mudbanks appear to form predicatable patterns defined by the interaction of depositional and early diagenetic sedimentation processes.
KeywordsTrace Fossil Sedimentary Petrology Depositional Unit Depositional Facies Seagrass Cover
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