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Introduction

Carbonate rocks are made of particles (composed >50% carbonate minerals) embedded in a cement.  Most carbonate rocks result from the accumulation of bioclasts created by calcareous organisms. Therefore carbonate rocks originate in area favoring biological activity i.e. in shallow and warm seas in areas with little to no siliciclastic input.  In present day Earth these areas are limited to ±40 latitude in region away or protected from erosion-prone elevated continental areas.

In the Devonian high-sea level, favoring the development of shallow epicontinental sea, warmer conditions, and relatively low detrital sedimentation rates, resulted in larger thick carbonate buildups.  The Murrumbidgee serie is an example of such carbonate buildup.

Because they contain numerous bioclasts (i.e. fossils) carbonate rocks are cherish by stratigraphers and paleontologist alike as they enable them to reconstruct stratigraphic sequences and make large-scale correlation between geological formations hundred to thousand of kilometer apart.  In addition carbonate rocks’ texture and the nature of their bioclasts offer a detailed insight into their depositional environment.

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Depositional Environments

Carbonate depositional environment past or present fall into three general types:

  • Ramp continental margins are continental platform gently sloping toward the ocean (<1).  They are limited by the emerged continent and toward the oceanic basin by the gentle continental break.  It is an environment of high energy as the amplitude of the waves increases as the depth of the sea decreases.
  • Rimmed margins are continental platform limited toward the ocean by a steep and abrupt continental break where a nearly continuous carbonate rim or barrier develops.  These rims or barriers are wave resistant structure made of reef coral (alive or dead) and oolitic sand shoals.  Landward the rim/barrier is a low energy environment of “lagoonal” characteristics grading landward into a tidal flat.  In present Earth they can be recognized by colorful tourists snorkeling happily together.  Oceanward, the rim/barrier is a high-energy environment.  In this noisy, wavy and scummy environment it takes commitment, strength and a bit of an attitude to any living organism to hold on onto the place.  Tourists are rarely seen in those dangerous waters.
  • Isolated platforms, also known as “Bahama type”, are shallow platform 10’s to 100’s km wide offshore of shallow continental shelves, surrounded by 100’s to 1000’s m deep water.  This environment is characterized by the absence of siliciclastic input.

 

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Dunham Classification

The Dunham classification (1962) is based on concept of grain/mud support therefore on the proportion mud-particle and the depositional textures. The concept of "support" assume continuity of either the mud matrix or that of the grains. If the carbonate is mud-supported the grains float into a continuum of mud matrix. In grain-supported carbonate rocks the grains from an interconnected skeleton in which the mud fills the gap. The percentage mud/cement at which there is a switch between mud-supported and grain-supported depends on the fabric (preferential agencement) of the particles.

DEPOSITION TEXTURE RECOGNIZABLE UNRECOGNIZABLE
ORIGINAL COMPONENTS NOT BOUND TOGETHER DURING DEPOSITION ORIGINAL BOUND
CONTAIN MUD, CLAY, AND FINE SILT-SIZE CARBONATE NO MUD
MUD SUPPORTED GRAIN-SUPPRTD
GRAINS < 10% GRAINS > 10%
MUDSTONES WAKESTONE PACKSTONE GRAINSTONE BOUNDSTONE CRYSTALLINE

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Mudstone

A mud-supported carbonate sedimentary rock containing less than 10% particles of clay and fine silt size; the original components are not bound together during deposition.

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Wakestone

Mud-supported limestone containing more than 10% carbonate grains (larger than 20 microns) "floating" in a finer-grained lime mud matrix.

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Packestone

Clastic limestone supported by its own grains, but also containing some calcareous mud.

Examples from Murrumbidgee...

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Grainstone

Clastic, grain-supported, mud-free limestones.

Here is a nice example of grainstone from Shark Bay (WA).

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Boundstone

Limestone formed of encrusting or reef-building organisms.

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Crystalline

Recrystallized limestone in which the original carbonate grains have been transformed in calcite or dolomite.

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| Info | Carbonates | Structures | Devonian | Contacts | Home |
Introduction
Depositional Environ.
Classification
Mudstone
Wakestone
Packestone
Grainstone
Boundstone
Crystalline