Sediment Supply and Structures

Sediment supply and sedimentary structures

(after Playon and Kerrans, 2007)

Types of carbonate Slope Sediments
Sediment found on carbonate slopes have a variety of origins and character:
  • Pelagic: particles typically less than a few micrometers, primarily consisting of calcareous and siliceous skeletal remains of marine phytoplankton and zooplankton.
  • Platform: platform carbonates transported off-platform transport in varying proportions of mud-sized algal and inorganically precipitated aragonite needles, mud to sand sized skeletal and nonskeletal debris, lithoclasts and bioeroded particles. Coarser sediments, including gravel and bolder sized lithoclasts, may originate from shallow-water facies or weathered carbonate bedrock.
  • hemipelagic: These sediments are fine-grained terrigenous materials that enter the marine system and are derived from coastal erosion or fluvial transport across the shelf to acumulate and on the slope to be thoroughly mixed with carbonate sediment.
  • Autochthonous carbonates: These sediments include carbonates that formed in-situ, e.g., fecal pellets of epifauna and infauna, seafloor Mg-calcite cement, peloidal Mg-calcite in foraminifera tests, and skeletal debris associated with the fauna of the slope setting.
The relative proportions of different sediments vary spatially according to different controlling factors, including:
  • Proximity to continent
  • Productivity of shallow water carbonate organisms
  • Latitudinal changes in tectonic plates which may effect carbonate production
  • Evolution of reef- building organisms
  • Locus of deposition of platform-derived sediments
  • Oceanographic setting
  • Sediment supply
 
sedimentary structures
There are a variety of sedimentary structures associated with the different areas of a carbonate slope.  These include:

1) Bouma sequences:

Idealy and characteristically these turbidite deposits are formed by successions of layers that from top to bottom include:
  • Pelitic or mud layer
  • Parallel laminations
  • Ripple laminations
  • Parallel laminations
  • Graded coarse sediment
Four Bouma sequences  (Coniglio and Dix, 1992)
 

2) Debris Flows:
Cohesive flows of sediment with matrix strength and grains the size of gravels

 
 
Graded conglomeratic flow  (Coniglio and Dix, 1992)
 

 
Boulders and blocks up to 10 feet in diameter surrounded by mud flow material.
(Scholle et al., 1983)

3)  Flute Casts:
These structures are formed by the filling of an erosional scour.  Flutes have spoon-shaped forms; their up-current end is bulbous and it flares in the down-current direction.

 
Flute Cast
http://www.geo.duke.edu/ss/ss.htm
 
4) Groove casts or Tool Markings:
These sole markings have rounded ridges, and form from the filling of grooves.  Shells, grains, pebbles and logs swept over firm muddy bottoms by currents probably produce most grooves. They are preserved as casts protruding from the lower surfaces of overlying beds.

Groove cast:

http://www.geo.duke.edu/ss/ss.htm

          

 
 
Sole Markings:
 

http://www.geo.duke.edu/ss/ss.htm

 
        
Load casts:
http://www.geo.duke.edu/ss/ss.htm
 
5) Glide surfaces:
These erosional features are observed within thinly bedded mud.  They are interpreted to form as a result of large-scale slides or slumps and to have little internal deformation.They are normally recognized by their sharp contacts.
 
 
Glide plane identified in outcrop:
(Brown and Loucks, 1993)
 
6)  Mud Mounds:
Mounds of skeletal debris and lime mud enclosed in beds of dark laminated sandy beds, or shales.

7)  Medium-scale cross-bedding:
Cross beds up to ½ mm thick, caused by scour and fill of megaripples formed by moderately strong currents.
 
 
 
 Cross bedding:
http://www.geo.duke.edu/ss/ss.htm
 
8)  Injection Dikes:

beds that cut vertically through massive or normally bedded strata, filled by material squeezed up from below in response to loading of the substrate by the overlying carbonate mass.

9) Filled fissures:
Discordant sediment filling fissures that cut across normally, bedded strata.  These fissures may have opened in response to slumping and/or tectonic activity.  The sediment fill is normally sand sized or coarser material.

10)  Nodular bedding:
Layers that consist of fine beds grading to loosely packed nodular bodies of rock in matrix of like or unlike character.

Nodular chert:
 http://www.geo.duke.edu/ss/ss.htm

11)  Sedimentary Boudinage:
These features form as the result of differential compaction of patchy deposits of shale and carbonate.  These deposits form irregular, closely spaced structures caused by a disruption of layers by stretching and flowage.

 
Boudin Structures
http://www.geo.duke.edu/ss/ss.htm

12) Bioturbation and Burrows:
Mixing of sediment by burrowing organisms.  Preserved as tubular ichnofacies whose margins may be emphasized through dolomitization.

 

Burrows on bedding plane:
http://www.geo.duke.edu/ss/ss.htm
 
 
 
 

Deposit Homogenized through bioturbation:
http://www.geo.duke.edu/ss/ss.htm
 

 

 
 
Intensive burrowing:http://www.geo.duke.edu/ss/ss.htm

 

References and Sources:

Bates, R.L., Jackson, J.A. Dictionary of Geological Terms.  Anchor Books, New York : 1984.

Brown, A.A., Loucks, R.G. Influence of Sediment Type and Depositional Processes on stratal Patterns in the Permian basin-Margin Lamar limestone, McKittrick Canyon, Texas in carbonate sequence stratigraphy:Recent Developments and Applications. Ed. Loucks, R.G. and Sarg, J.F.  AAPG Memoir 57, Tulsa: 1993. p.145.

Coniglio, M., Dix, G.R. carbonate Slopes in Facies Models: Response to Sea Level Change. Ed. R.G. Walker and N. P. James. Geological Association of Canada, St.Johns: 1992. pp 349-373.

Scholle, P.A., Bebout, D.G., Moore, C.H., (Eds.). carbonate Depositional Environments. AAPG Memoir 33, Tulsa: 1983.

Wilson, J.L. carbonate Facies in Geologic History. Springer-Verlag, New York: 1975.
 


Ancient Slopes


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