Morphogenesis and facies relationships of thrombolites and siliciclastic stromatolites in a Cambrian tidal sequence (Elat area, southern Israel)

Thrombolites and siliciclastic stromatolites occur within the Lower Cambrian marine Timna Formation in the Elat area, southern Israel. This rock succession consists of four consecutive stratal suites which together represent a transgressive-regressive cycle within a peritidal depositional system. The thrombolites are associated with the sea-ingression stage and occur as a series of massive-bedded strata with laminoid fenestral layers and with a microclotted-peloidal fabric of the micritic material. SEM-EDS study of the clotted and peloidal micrite reveals a calcitic framework of pitted polygonally shaped units, 2 μm wide, together with a few groups of compressed, large hollow structures. These forms are interpreted as the possible remnants of bacterial decomposers and of coccoid-dominated mat-forming unicells, respectively. The intimate structural connection between the carbonate fraction and the microbe structures suggest in situ carbonate production through bacterial decay of the cyanobacterial material during its self-burial, with carbonate precipitating as solid phases on vacant extracellular envelopes. The siliciclastic stromatolites developed during seashore progradation and now occur as rare, laterally linked columnar forms within channelled cross-stratified sandstones at the top of the Timna succession. They are finely laminated with a low to moderate degree of inheritance of the laminae and are partly burrowed by Skolithos. Textures and the geometry of the siliciclastic stromatolites suggest that they were formed through binding of transported quartz sand-sized particles by bottom-dwelling filamentous microorganisms. nt production and the growth-pattern of the two structures within the Timna sequence seem to have been regulated by different microbial assemblages which shifted in response to changes in environmental factors through time and space. The differential distribution of the two buildups in the sedimentary sequence reflects the difference in balance between the biological attributes of the two biotas and the physical environment. Pitted architectures, such as those displayed by the Timna thrombolites, could be an ontogenetic fingerprint of microbially constructed rocks, which might help to recognize them in the fossil record when other evidence is lacking.