Clastic and Carbonate Petrology

Clastic and carbonate rocks are the two dominant families of sedimentary rock, distinguished by whether their grains are transported detritus or chemically and biologically precipitated carbonate.

Definition

The petrographic study of clastic sedimentary rocks built from transported mineral and rock fragments and of carbonate rocks formed chiefly by chemical and biological precipitation of calcium carbonate.

Scope

This topic covers the composition and texture of clastic rocks, framework grains, matrix, and cement, and their maturity, alongside the constituents of carbonate rocks, ooids, peloids, bioclasts, and micrite, and the processes that form them. It compares the contrasting origins of detrital and chemical-biochemical sediments.

Core questions

• What are the framework grains, matrix, and cement of a clastic rock?
• How do sorting and rounding express transport history?
• What are the principal carbonate grain types and how do they form?
• How do clastic and carbonate systems differ in their controls?

Key theories

Clastic framework, matrix, and cement

Clastic rocks consist of detrital framework grains, fine interstitial matrix, and chemically precipitated cement; their relative proportions and grain characteristics record provenance, transport, and diagenesis.

Carbonate grain and mud constituents

Carbonate sediments are built from biologically and chemically produced grains, ooids, peloids, bioclasts, and intraclasts, set in lime mud, with composition reflecting the in-situ marine environment rather than external source rocks.

Clinical relevance

Detailed petrography of clastic and carbonate rocks is central to evaluating reservoir quality for hydrocarbons and water, reconstructing depositional settings, and interpreting the diagenetic alterations that affect porosity and permeability.

History

Mid-twentieth-century petrographic work distinguished the fundamentally different origins of clastic and carbonate rocks; carbonate petrography in particular advanced through thin-section atlases and the recognition that carbonates form largely in place from biological activity.

Key figures

• Robert L. Folk
• Maurice E. Tucker
• Peter A. Scholle

Seminal works

• boggs2009
• tucker2001
• scholle2003

Frequently asked questions

Why are carbonate rocks considered intrabasinal?

Most carbonate sediment is produced within the basin of deposition by organisms and chemical precipitation, rather than being eroded from distant source rocks like clastic detritus.

What is cement in a sandstone?

Mineral material, commonly quartz, calcite, or clay, that precipitates in the pore spaces between framework grains during diagenesis and binds the rock together.