Interpreted scans of thin-sections of specimen S3.2. (A) Transverse cut and (B) longitudinal cut. Interpreted boundaries between growth zones are shown as black lines, and the position from which growth initiated is shown as a red line. Zones 1-6 are described in the text. Unlike wholly abiotic stalactites, this specimen is strongly asymmetrical, caused by calcifying cyanobacteria favoring the illuminated side of the specimen. Images are shown at the same scale. (Color graphics available at www.liebertonline.com/ast) 

Contexts in source publication

Context 1

... S3.2 is a downward-tapering stalactitic rock (Fig. 4) that was collected as a float specimen that was found adjacent to the central barrage outcrop in the quarry (Fig. 2B, 2C). Internally, there is strong asymmetry, and at least six separate components of the visible fabric of the sample, referred to here as Zones 1-6, were delineated (Fig. ...

Context 2

... 2 ( Fig. 5) consists of micrite intergrown with dark brown sparry calcite fans, with the latter being dominant. Each fan is around 1-3 mm across, and all fans grew toward the outward edge of the specimen. This zone forms a band around 1 cm thick on one side of the specimen (the side toward which Zone 1 micrite prograded) and is traceable ...

Context 3

... 4 is recognizable in hand specimen only as a white band that separates brown calcite below it from very similar- looking brown calcite above, which is designated as Zone 5. Note that Zone 4 appears brown in thin-section (Fig. ...

Context 4

... 4 was recognized petrographically by the re- appearance of micrite on one side of the specimen (Fig. 5B). This zone is laterally traceable into spar via two c. 5 lm thick subparallel micritic bands that mark its top and bot- tom. The micrite comprises a porous network of peloids that includes hollow tubes of c. 130 lm ...

Context 5

... complex growth history of specimen S3.2 was un- raveled on the basis of the petrographic analysis, with dif- ferent growth zones exhibiting different degrees of biotic influence. The oldest part of the specimen (in Zone 1; Fig. 5A, 5B) comprises dense, white-colored calcitic tufa that contains clusters of hollow carbonaceous tubes of fossilized cyanobacterial trichomes preserved inside sheaths. Zone 1 was formed by calcification of filamentous cyanobacterial shrubs that coated an overhanging leaf or twig. This pho- tosynthesizing biofilm formed a substrate for ...