Petrographic microfabrics of Upper Jurassic Smackover thrombolites: insights into clot formation

Thrombolites are defined by their clotted fabrics, yet the origin and significance of those fabrics remain unresolved. Decades of study have produced multiple, often contradictory, hypotheses, and recent work increasingly suggests that similar “mesoclots” may form through fundamentally different biological and diagenetic pathways. The current classification schemes are limited and mix fabric-based and process-based categories, often obscuring insight into what the fossilized clot represents or how clotted fabrics form relative to laminated microbialites. Without clear mechanistic constraints, the interpretive value of thrombolites as paleoenvironmental archives remains limited. We use petrographic and organic biomarker analyses of Upper Jurassic Smackover Formation thrombolites from the Little Cedar Creek Field, Alabama, to test competing models of clot formation and fossilization in one of the last globally extensive Phanerozoic thrombolite reef systems. Our observations show that mesoclots lack any internal lamination and are composed of dense micrite intergrown with micritized grains and skeletal components, with fabrics that vary along a continuum rather than discrete types. Constructional features and biomarkers indicate that clotted textures developed during mat growth around microbial and eukaryotic communities in slightly restricted marine conditions. Placed in the context of ancient and modern thrombolites, our results establish the Smackover examples as one category within a broader, process-based framework that links clotted fabrics to growth mechanisms and defines operational criteria based on fabric, mineralogy, and geochemistry to distinguish among multiple thrombolite types.