Torosites: A New Microbially Mediated Speleothem from Sistema los Toros, Nuevo Léon, Mexico

Abstract

Although several kinds of speleothems are known to grow with pendant morphology (i.e., helictites, pool fingers, and stalactites), we describe here a new form found in Sistema Los Toros, Nuevo Léon, Mexico, which we propose to call torosites. A laminated, bladed, spar crust grew beneath remnants of a stromatolitic micritic crust and extended mostly downward as irregular finger-like protrusions (torosites) that are 2–3 mm wide and 2–3 cm long. Although mainly pendants, the individual torosites are quite irregular with growth sideways and even upward in places. The torosites thicken and thin, like knuckles on a finger, but maintain a relatively uniform diameter. Laminations in the bladed spar and protrusions are defined by micritic intervals with minor intermixed detrital grains (clay, quartz, and carbonate silt) and rare, ropy biofilm. Bladed-spar crystals nucleate off thicker detrital layers, but they extend through thinner lamina. The torosites lack the central canal of helictites and most stalactites. Unlike pool fingers and stalactites, torosites only grow on the end of the speleothem, not on the sides. We hypothesize that the torosites grew from dripping water at their ends. Microbial biofilm communities colonized the growth surface, probably during drier conditions, forming the micritic layers. We speculate that the microbial mat likely caused the irregular sideways growth, but we cannot identify how on these fossil forms. Torosites are unique and add to the growing list of known microbially mediated speleothems.