Bridging the gap: Anatomy of recently growing calcareous crusts in the Bohemian Karst (Czech Republic)

This study presents the results of a combined field and microscopy investigation of an unusual hybrid type of carbonate crust that precipitated in subaerial outcrops of Lower Palaeozoic limestone in the Bohemian Karst. The crusts formed pendant, up to 6 cm thick, stalactite-like deposits that filled various cavities within fractured limestone beds. Two distinct microfacies were identified in the crusts: highly porous, coarsely crystalline aggregates that host abundant moss stems, interpreted as bryophyte tufa, and tiny, hemispheroidal, mushroom-shaped coralloids. In some of the thicker crusts, both microfacies coexist in close proximity. However, more commonly, the bryophyte tufa is found in open tectonic fractures and dissolution grooves along limestone bedding planes in sunnier areas of the outcrops, while coralloids dominate in darker, wetter environments such as small crevice caves and the shadowed undersides of overhanging limestone beds and flat limestone cobbles.

The coralloids exhibit a distinct, fine internal lamination consisting of convex outward, rhythmically alternating dark and light-colored calcite laminae. The dark laminae are enriched with clay minerals, organic debris, detrital grains, and fine FeMn pigments, which are derived from the limestone bedrock, nearby relict terra rossa paleosoils, and the overlying rendzinic soil cover. These materials were concentrated during thaw or heavy rainfall events. Cathodoluminescence (CL) microscopy of the coralloids revealed vividly luminescent zoning, characteristic of abrupt changes in the composition and reducing potential of the seeping water, which cyclically repeated during speleothem growth. Characteristic peloidal clotted textures, indicative of microbial influence, were also observed in many laminae. This, along with the stromatolitic character of the lamination and the presence of various microorganisms—including actinobacteria, algae, fungi, and bacteria identified on the coralloid exteriors—collectively suggests that the formation of these speleothems likely involved biologically mediated phases.

Surface calcite rhombs on the calcareous crusts were corroded, exhibiting well-developed etch pits, and spiky and ribbon-like calcite crystallites exposed within the cores of severely etched crystals. These erosional effects, likely due to bacterial or fungal activity, suggest that the longevity of these deposits in the rock record may be limited. This is further supported by the relatively young age of the crusts (30 to 70 years), determined through radiocarbon dating and field observations. It is proposed that these recently growing calcareous crusts represent a hybrid, transitional form of carbonate deposits, bridging the gap between biologically influenced calcareous bryophyte tufa and typical speleothems, akin to those recently discovered in the twilling zone of some tropical caves.