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
•
•
•
•
•
•
•
Example: Bahamas
Subtidal zone
Surf zone
Channeled tidal flats with algal mats
Supratidal marsh
Eolian dunes
Transgression leads
to cyclic shallowing upward sequences
Normal marine tidal flats
•
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.
•
•
•
•
Fringing reefs
Tidal mudflats
Oolitic shoals
Grapestone regions represent less protected, zones of lower accumulation rate
Great Bahama Bank, 1
•
Great Bahama Bank, 2
Area of 100,000 km2
• 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.