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Shallow marine carbonate environments

By Kenneth Shaw,2014-04-08 21:22
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Shallow marine carbonate environments

Shallow marine carbonate environments

     Tidal

     Subtidal

     Reefs and bioherms

    The shallow water carbonate factory

    Environment

     Warm water

     Clear (low nutrient)

     Saturated carbonate ion

     Low clastic load

     Growth to shelf margin

    Tidal carbonate environments In situ carbonate mud production rather than transport of

    sediments

     Rapid growth, widespread in past

     Occur on protected coasts sheltered from wave energy

     Exhibit shallowing-upward sequences

    Tidal carbonates, 1 Occur on wave protected coasts

     On wide, flat, shelves far from the shelf break

     Behind a steep carbonate beach face

     Behind a barrier Isle of carbonate debris

     Behind oolitic Sand Banks

     Behind a reef

     Important to keep in mind that carbonate tidal flat and reef environments

    form one continuous environmental setting

    Tidal carbonates, 2 Widespread in ancient epeiric seas

     Very high carbonate production rates

     Thus very difficult to “drown” a carbonate environment during marine transgression

     Modern analogs for two primary classes: Normal marine flats with tidal channels

    (Bahamas)

     Evaporitic tidal flats (Persian Gulf)

     Both are time-transgressive environments

    Normal marine tidal flats Example: Bahamas

     Subtidal zone

     Surf zone

     Channeled tidal flats with algal mats

     Supratidal marsh

     Eolian dunes

     Transgression leads to cyclic shallowing upward sequences

Regression can result in stressed environments

    Subtidal and reef carbonates

     Provide barrier to tidal carbonate environments Oolitic shoals

     Reefs

     Bioherms (Build ups)

    Oolitic sands, Bahama Bank

    Modern oolitic sands

    and stromatolites

    Subtidal shelf carbonate Below mean wave base to depths of 100-200 m

     Normal marine conditions

     Mixture of skeletal sand and mud

     Less regular bed thickness than tidal flats; flow structure present Intensive bioturbation

     Modern examples

     Florida Coast

     Persian Gulf

     Yucatan Coast

     Bermuda

     Bahamas

     Great Barrier Reef

    Ancient examples

     Carbonate production can keep pace with slow subsidence or sea

    level rise

     Tremendous thickness: 2-3 km sediment Cretaceous seaway of North America, Mexico Triassic Dolomites of the Italian Alps Example:

    Sediments

    of an epicontinental sea

    Cretaceous Seaway

    of N. America

     Sea level high stand (Ice-free conditions?) Marginal Marine - Marine sediments

Vast epicontinental sea-

     5000 km long by 1400 km wide

     Approximately 300 m at greatest depth

     2200 m of marine sediments

    Depositional environments

    and associated sediments

     Deep water - pelagic shale and chalk

     Shelf - Rim of shale and sandstone

     Coastal plain deposits - terrestrial shale, sandstone and conglomerates Transgressive/regressive facies changes in response to SL changes and orogeny of Rocky mountains.

    Great Bahama Bank, 1

     Fringing reefs

     Tidal mudflats

     Oolitic shoals

     Grapestone regions represent less protected, zones of lower accumulation rate

    Great Bahama Bank, 2

    2 Area of 100,000 km

     Average water depth 5m

     Oolitic shoals indicate high energy conditions Exports large amount of aragonitic mud to the deep sea

    Reefs and Buildups

     Buildups - any bio-precipitated carbonate rock with topographic relief above surrounding environment

     Reef - buildup of a wave-resistent framework within the wave zone Bioherm or biostome - in situ accumulation of non binding benthic calcifers

     Framework builders account for only ~10% of Reef!

Geometry

     Fringing

     Barrier

     Atoll

     “Spur and groove”

    Reef type sequence

    Spur and groove on Fringe reef in Taiwan

    Vertical zonation of reef environments

    Progradation and aggradation

    on carbonate margins

    Modern carbonate environments

    Sources of limestone sand & mud Coralgal sand on outer reef

     Fine grained carbonate in back reef

    Evidence for in situ production

    of back reef lime muds

     Composition of mud from modern lagoon/tidal flat environment differs from reef carbonate Data indicates calcitic algae grow at sufficient rates in back reef environs to explain recent sediment accumulation

     Typical lime mud production rates are similar (10’s to 100’s of cm/kyr) although reef environment may have rates up to 2x greater than lagoon/tidal flats

    Limestone facies and depth ranges

    Vertical zonation of coral species Reef crest (~1-6m)

     A. palmata

     Forereef (~3-12m)

     A. cervicornis

     Deep water (>12m)

     Monastrea, Siderastrea, Diploria sp.

    Example of ancient reef facies

    Reef builders through time

    Sample exam question

    Q. Modern observations demonstrate that the coral species A.

    palmata grows over a depth range from ~1-6 meters. Yet Late Pleistocene reef deposits from the Caribbean often include in place

    layers of A. palmata 20-30m thick. Describe three hypotheses that might explain these results.

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