How timely! This article covers an important part of "The Land That Fossils Built" that I uploaded to the Geology2 Files section recently. The first part of TLTFB describes the fossils in northern Arkansas, but parts 2 and 3 are concerned with the numerous thrombolite reefs there and the major part they played in shaping the environment during the late Mississippian Period. I've included a number of pictures of mottled thrombolite textures along with some interpretations. I also touch on the relationship between thrombolites and stromatolites in the following passage:
"Stromatolites are known from rocks ranging in age back to the early Precambrian, but thrombolites did not appear until the late Precambrian when cyanobacteria began to precipitate fine-grained calcium carbonate. Before that point, all stromatolites were made of coarse-grained sediment, but afterward began to occur as both coarse-grained and fine-grained types. The thrombolites were most abundant during the Cambrian and Ordovician and were much larger in the past than those that exist today. The habitats of both stromatolites and thrombolites have continued to dwindle until the present when they are known from only a few places in the world. The evolution of grazing sea bottom organisms must have had something to do with this. One expert observed that the microbial communities that form these structures may be the slowest evolving and longest declining populations on earth."
The Arkansas thrombolite reefs show stromatolitic laminations in only a few places. However, there is a series of limestones in Spain that contain alternating layers of thrombolites and stromatolites through hundreds of feet of vertical section. The first thing that jumps to mind is that this would result from cyclic changes in water depth, but no one has ever been able to link water depth as a factor by independent evidence.. Stromatolites and thrombolites are found in the same general environments today and the physical conditions that govern their separate occurrence is still a puzzle.
The thrombolite reefs in northern Arkansas had a tremendous influence on the environments around them. They provided a stable surface surrounded by an environment of sand that shifted continually, and as such they were home to a multitude of plants and animals, most of which disintegrated after death and fed the sediment accumulation that built up raised banks on the shallow sea floor. They channeled the shallow tidal currents and created sheltered areas on their landward sides adapted to special fauna, Ultimately, the topography they created on the sea floor would be reflected in the present topography along a narrow zone stretching across northern Arkansas where oddly shaped peaks and ridges are common. Topo maps and aerial photos of these features are shown in part three of TLTFB.
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