Origin and significance of tube structures in Neoproterozoic post-glacial cap carbonates: Example from Noonday Dolomite, Death Valley, United States

The Neoproterozoic Noonday Dolomite (Death Valley, USA), a post-glacial cap carbonate, contains closely packed, meter-long, cm-wide, tube-like structures that define the vertical accretion direction. Similar tubestones are known from post-glacial cap carbonates in Namibia and Brazil. In vertical cross section, the tubes average 2 cm in diameter, pinch and swell greatly along their length, may bifurcate and coalesce, and are filled with brown laminated micrite/microspar where best preserved. The tubes do not root or terminate in a particular layer and are randomly distributed where present. The laminated host rock is composed of an early lithified, microclotted fabric with framework void space filled with sparry dolomite cement. The contact between the tube fill and the host rock is diffuse and feathered; commonly, wisps of laminated host rock cross the tube fill and bridge between adjacent stromatolitic structures, compartmentalizing the tubes. The tubes likely result from the contemporaneous interplay between microbialite growth and sedimentation/cementation, rather than fluid or gas escape, as demonstrated by the compartmentalization by bridging laminae. Vertical cross sections resemble inter-column depressions that form between columnar stromatolites. Bed-parallel sections, however, reveal that the tube structures represent isolated, sediment-filled depressions within a continuous layer of stromatolite. The genesis of this unusual stromatolite morphology is likely related to highly supersaturated seawater in the aftermath of low-latitude glaciation in Neoproterozoic time. Similar tube-forming microbialites are known from alkaline lake systems such as Lake Turkana, Pavilion Lake, and paleo-Lake Gosuite (Green River Formation). The tubestones are interpreted to represent a rarely attained end-member in stromatolite morphospace, likely associated with anomalously high carbonate supersaturation.