ENIGMATIC "TUBE-LIKE" STRUCTURES IN THE NOONDAY DOLOMITE: CONSTRAINTS ON POSSIBLE HYPOTHESES

Textural and chemical data were used to constrain the origin of tube-like structures in the large, laminated domes of the Neoproterozoic Noonday Dolomite: 1.The tubes are generally blind and do not form a thoroughly interconnected network. 2. The tubes were likely formed at the surface since the best-preserved examples are filled with allochthonous sediment. 3. The contact between the tube and host rock is diffuse, indicating that the tubes were not destructively emplaced into the host rock. 4. The tubes are oriented vertically, regardless of their position on the domes. 5. The tube-fill and host rock are isotopically homogeneous.

The three most reasonable previously proposed hypotheses were tested using these constraints: fluid escape, karst, and interstromatolite fill.

The fluid escape structures hypothesis fails to meet all five constraints. In particular, known escape structures commonly show evidence for destruction of the host rock, in marked contrast to the diffuse tube margin noted here. In the special case of methane escape, one would expect botryoidal carbonate cements with d¹³C values of approximately -30 to -70‰ PDB to line the void spaces; this is not noted in the Noonday Dolomite.

The karstification structure hypothesis fails to meet the first and third constraint since the structures do not root down from a karst surface and dissolution would imply destruction of the host rock. Dissolution surfaces are often characterized by a buildup of insolubles and sharp interfaces (not noted here).

The final hypothesis would suggest that the tubes are interstromatolite spaces. This mechanism provides for the formation of the spaces between stromatolites not only within the dome, but also at the surface of the dome. Furthermore, this mechanism is constructional, not destructive in nature. Since stromatolites build upward towards sunlight, the growth of the spaces between them has an upward motivation as well. Lastly, this hypothesis allows for the host-rock and tube-fill to be precipitated at the same time and in the same seawater conditions, thus preserving isotopic homogeneity. Therefore, the interstromatolite spaces hypothesis is not falsified by the evidence found in this study.