Classic Cambrian Thrombolite Clots Formed by Faint Ministromatolites

Thrombolite — microbial carbonate with macroscopically clotted fabric — is the second most widely recognized microbialite category after stromatolite. Thrombolite clots can include calcimicrobes, algal-foraminiferal colonies, dense micrite, and peloids, but their construction mechanisms often largely remain unclear. To assist understanding, we re-examined the original specimen on which the term thrombolite is based. Restudy of James Aitken’s classic late Cambrian (Furongian) thrombolite from western Canada shows that the individual clots (2–10 mm wide, up to 15 mm high) are delicately laminated in thin section. Thrombolites with similar finely laminated clots occur in the middle Cambrian of Inner Mongolia, China, and in the late Cambrian of Texas, USA. If these laminations were evident to the naked eye, these clots could be described as ministromatolites (small stromatolites, ≤ 20 mm wide). Since this lamination is not visible to the unaided eye, these finely laminated structures can be regarded as thrombolite clots at hand-specimen scale. We propose that these Cambrian examples are part of a much longer record that connects Proterozoic ministromatolites/microdigitate stromatolites to present-day examples. Distinctly laminated ministromatolites/microdigitate stromatolites are common in Paleoproterozoic–Mesoproterozoic (2.5–1.0 Ga) shallow-marine carbonates. During the Neoproterozoic, rare reports suggest that their lamination became inconspicuous, grading into the faintly laminated clots of classic Cambrian thrombolites. At Lake Clifton, Western Australia — a widely cited Holocene thrombolite analog — the clots formed by calcified cyanobacterial colonies closely resemble these Cambrian examples, and show initial lamination obscured during early diagenesis. We suggest that ministromatolites record a ~2.5 Gyr history of fluctuating marine calcification, broadly tracking long-term decline in atmospheric CO2 and seawater carbonate saturation. In all these cases, the distinction between ministromatolites and the thrombolite clots that they formed is fundamentally resolution-dependent.