Paper No. 9
Presentation Time: 8:00 AM-12:00 PM
EXOTIC BIOGENIC STRUCTURES FROM EARLY CAMBRIAN OF THE MARBLE MOUNTAINS, CALIFORNIA
Three previously unobserved biogenic structures from Girvanella limestone of the Chambless Limestone (CL) and silty carbonate mudstones of the overlying Cadiz formation provide new insight into diversity of the Early Cambrian benthic communities. The first of these biogenic structures consists of tube-like penetration structures that extend from the outer edge to the center of the large oncoids near the base of the CL. There may be as many as three of the tube-like structures in a single oncoid. It is uncertain whether this structure represents some type of boring or a bioclaustration. If these structures proved to be a bioclaustrations, they would be the earliest record of bioclaustrations. The second biogenic structure encrusted hyolith shells, occurs in the oncoids from the Cadiz formation. The absence of the hyolith opercula suggests that the oncoid growth began after the death of the hyolith. The presence of a passageway through the oncoid laminae to the interior of the hyolith shell indicates that some, yet to be identified organisms inhabited the hyolith shell while the oncoid was enveloping the shell. The third structure is the cup-like structure (CLS) that occurs on the surface of minor karstic disconformity at the base of the CL. The CLS occurs in a skeletal grainstone and are ~ 8-9 cm in diameter with a raised periphery and may be as much as 10cm deep. A number of these CLS structures are observed, in one case, three of these structures occurred within a meter of each other. It is uncertain whether these structures represent a resting feature or a shallow burrow-like depression. It is clear that these structures were formed prior to the lithification of the skeletal grainstone. Following lithification and minor karstification of the upper surface of the grainstone, the structures were filled and preserved with large well-developed oncoids during the flooding event that led to the deposition of the CL.