Holding it together

Even for experienced geologists it is always exciting to come across direct and tangible evidence for a concept conceived in the 18th century Scottish Enlightenment, taken up by James Hutton and immortalised by Charles Lyell as “the present is the key to the past”. The most common are ripple marks, sun cracks and raindrop impressions, usually in sandstones. Now relatively high-energy currents move sands, so that every tide on a beach or in an estuary obliterates the previous tide’s sedimentary structures: it is easy to think of them as somehow being ‘one in a billion’ chance preservations. In fact they are a lot more common than common sense might suggest. That is because photoautotropic bacteria can coat sediment surfaces quite quickly to form biofilms or microbial mats, given the right conditions. They knit the grains together, thereby armouring the structures against erosion to some extent. The October 2008 issue of GSA Today begins with a useful summary of the influence of biofilms in preserving intricate signs of sedimentary processes (Noffke, N. 2008. Turbulent lifestyle: microbial mats on Earth’s sandy beaches – today and 3 billion years ago. GSA Today, v. 18 October issue, p. 4-9). Equally important, the author shows how close examination of Archaean littoral sedimentary structures reveals clear signs of the microbial mats themselves. These are convincing evidence for ancient life, even in the absence of tangible fossil cells (the oldest undisputed fossils date back only about 2 Ga).

The Palaeozoic record of sea-level change

Variations in global sea level shift the positions where different sedimentary facies are deposited, and also shift some aspects of oceanic chemistry. Consequently they have long been of interest to petroleum explorationists because reservoir and source facies will be laid down in different areas as the sea inundates stable continental areas or withdraws from them. Plots of changing sea level can be derived indirectly from seismic sections that reveal on- and off-lapping stratal sequences with detail added from the stratigraphy of such sequences determined in the field or from well logs, and considerable detail is available globally for the Mesozoic and Cenozoic Eras. The Palaeozoic Era is not so well known, and information has been acquired piecemeal but not correlated to time. So, a semiquantitative compilation will be welcomed in many quarters (Haq, B.U. & Schutter, S.R. 2008. A chronology of Paleozoic sea-level changes. Science, v. 322, p. 64-68). The outcome reveals a steady rise, with short term ups and downs, from about modern levels to around 220 m higher through the Cambrian and Ordovician, dropping in late Ordovician times by about 80 m, perhaps due to glaciation at that time. Through the Silurian and Devonian global sea-level stood around 180 m higher than now, with only broad fluctuation, then to fall gradually through the Carboniferous to reach modern levels around 320 Ma.  The Devonian to mid-Carboniferous decline marks the onset of the longest glaciation in Earth’s history, the lasted until the late Permian. The broad shifts have a superimposed short-term eustatic fluctuations, resolved into 172 separable events that vary in amplitude from a few tens of metres to around 125 m. Parts of the record show short-term fluctuations that may correspond to the ~400 ka cycle bound up with Earth’s orbital eccentricity. Yet there is insufficient evidence outside the Carboniferous-Permian glacial epoch to suppose that 400 ka shifts in sea level had a glacial origin


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