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

This study reports a well-developed columnar joint set in organic-rich carbonate source rocks, exhibiting distinct vertical prismatic columns with hexagonal to polygonal cross sections. These columnar joints are found within the Upper Cretaceous Muwaqqar Chalk Marl (MCM) Formation, in central Jordan. While columnar joints are commonly observed in igneous rocks, their occurrence in carbonate mudrocks is unique and has rarely been documented in stratigraphic records. This study aims to characterize the geometry and petrographic features of columnar joints observed in organic-rich carbonate source rocks and investigate the mineralogical and geochemical alterations associated with their development. Thorough petrographic and geochemical investigations were carried out to better understand the characteristics, paleoenvironmental context, and associated processes that controlled the formation of these honeycomb-shaped mudrocks. Our findings suggest that the observed columnar joints resulted from subsurface thermal alteration due to the endogenous combustion of the organic-rich carbonate source rocks. This combustion process has led to the development of joints oriented perpendicular to the bedding planes, extending throughout a 2-m-thick interval. These joints slightly taper downward while preserving their polygonal pattern, with column diameter decreasing from 3 to 1 cm. The development of columnar joints within the study interval is primarily attributed to volume contraction resulting from the consumption of organic matter during combustion and textural and pore-structure changes that occurred during recrystallization. This is corroborated by a significant reduction of total organic carbon (TOC) content, which reduced to less than 1 wt % in the combusted interval, compared to an average TOC of 15 wt % in the surrounding relevant strata. The findings of this study highlight a unique geological phenomenon that has not been previously widely documented in stratigraphic records.