REINTERPRETATION OF A PROTEROZOIC ENIGMA: JACUTOPHYTON AND THE SEQUENCE STRATIGRAPHIC DEVELOPMENT OF STROMATOLITE REEFS

The Proterozoic stromatolite Jacutophyton consists of low-relief, unwalled, columnar branches over a central, steep-sided, high-relief cone. Despite its stratigraphic importance in Riphean successions worldwide, the depositional origin of this complex stromatolite remains poorly understood. Here we present a detailed examination of lamina structure and sedimentological relationships in Jacutophyton from stromatolite reefs in the ~1.2 Ga Atar Group, Mauritania and reinterpret its growth within a sequence stratigraphic framework.

Stromatolite lamina represent the active growth surface of the mat and record the morphology of the depositional surface (synoptic relief), the relationship between mat growth and sediment deposition (wall structure), and the influence of wave and/or current action on mat growth (stromatolite elongation); lamina superposition thus records time-wise shifts in these parameters. In Jacutophyton, the high synoptic relief and height:width ratio of the central cone indicates deposition in a low-energy environment with little or no sedimentary influx, and absence of lamina erosion and interstromatolitic debris suggests growth below wave-base. These observations suggest cone growth during marine transgression and early highstand, when accommodation space is at its maximum and inhibits sediment influx to the basin. Low-relief, variably elongate branching elements in Jacutophyton, however, initiate from disrupted regions on the outermost laminae of cones and develop in concert with deposition of abundant rip-up clasts in intercolumnnar and interconical regions. These observations suggest branch development in a shallow-water, high-energy depositional environment and are consistent with growth during a fall in relative sea level, wherein wave-base impinges upon the central cone, impacting stromatolite growth.

In this scenario, Jacutophyton development is interpreted as a single, complete parasequence. Additional complexity in Atar Group stromatolites, such as the lateral juxtaposition of conical and branching forms and superposition of secondary cones atop branching elements, can be interpreted within this framework as resulting from stromatolite growth during discrete sea level phases in successive parasequences.