Geochemical Indicators of Organic Matter Combustion Governed by Pyrometamorphism in Columnar-Jointed Carbonate Source...

Area of Science:

• Geology
• Sedimentary Petrology
• Geochemistry

Background:

• Columnar joints are common in igneous rocks but rare in sedimentary strata.
• Organic-rich carbonate source rocks, like the Muwaqqar Chalk Marl (MCM) Formation, are crucial for hydrocarbon exploration.
• Understanding unusual geological formations aids in interpreting paleoenvironmental conditions and diagenetic processes.

Purpose of the Study:

• To characterize the geometry and petrographic features of columnar joints in organic-rich carbonate mudrocks.
• To investigate the mineralogical and geochemical alterations associated with columnar joint development.
• To determine the formation mechanism of these unique honeycomb-shaped mudrocks.

Main Methods:

• Detailed petrographic analysis of rock samples.
• Geochemical investigations, including total organic carbon (TOC) content analysis.
• Field observations of joint geometry and stratigraphic context.

Main Results:

• A well-developed set of vertical columnar joints with polygonal cross-sections was identified in the MCM Formation, central Jordan.
• These joints formed in a 2-m-thick interval, tapering downward with column diameters from 3 to 1 cm.
• Significantly reduced TOC content (<1 wt %) in the jointed interval compared to surrounding strata (average 15 wt %) indicates combustion.

Conclusions:

• The columnar joints in this carbonate mudrock are attributed to subsurface thermal alteration and endogenous combustion of organic-rich source rocks.
• Volume contraction during organic matter consumption and recrystallization during combustion led to joint formation.
• This study documents a rare geological phenomenon, expanding the known occurrences of columnar jointing in the stratigraphic record.