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Tidal signatures in an intracratonic playa lake

It was found by research carried out in the 1970s and 1980s that some sedimentary features appeared to be the results of tidal forces in tide-dominated marine environments. It was suggested at the time that these features could be related directly to astronomical forcing by the Sun and Moon. These sedimentary structures have since been found in many places around the world and were interpreted as indicating that the sediment was deposited in marine tidal environments. The authors1 report sedimentary features from the shores of Lake Eyre in Australia of the present, in an intracratonic playa setting that suggests the current paradigm, that these structures indicate a marine tidal influence, may in some cases where there is no other supporting evidence for a marine environment, may actually have been formed as in the case of the Lake Eyre structures. The authors1 have attributed these features in Lake Eyre to meteorological tides that are responsible for generating daily changes in wind directions and velocity, as well as variations in discharge of the feeder river systems, the first instance to be known of where such sedimentary features occur together in a continental setting. As a result of this finding the authors1 have suggested that the tidal sedimentary structures that have been recognised in ancient rock successions should not be attributed automatically to tidal currents resulting from the astronomical or marine tidal currents now that similar features have been found in continental environments. They1 also suggest that structures in ancient rocks that have been interpreted as marine tidal in origin but have no other supporting evidence that they are indeed of marine origin may need to be reevaluated, playa lakes and shallow lacustrine settings being considered in the interpretation of strata that have formerly been interpreted as of classic marine tidal origin.

As part of a study of sedimentary environments associated with endorheic, i.e., closed, drainage basins, the authors1 traveled to Lake Eyre in July 2011. Their study was concentrated in terminal splays, the fans at river mouths that sometimes trickle into ephemeral lakes. When they dug a trench at the mouth of Kalaweerina Creek they found flaser bedding, wavy bedding and lenticular bedding. These 3 types of deposit form when there are different ratios of mud and sand and when the energy levels of the water vary, as occurs in ebb and flow of coastal tides. When there is more sand than mud in a high-energy environment flaser bedding forms. In calmer environments, when there is more mud than sand, lenticular bedding forms. According to Hasiotis1 these are "textbook" tidal marine deposits. According to the authors1 it was the first time any of them had seen these structures and packaging in continental environments, especially in the setting of a dryland water body in which there is no water body for most of the time. They found that changing winds on this shallow land-locked lake and variable discharge from the stream is capable of producing the same effect.

As they worked they could see the process taking place, early in the day the wind was blowing the water body in one direction at the mouth of the creek, then later in the day the wind changed and blew the water in the other direction. They say the sediments were deposited in the last few hundred years, in the lifetime of the landlocked lake, so it was not possible for the structures to be mistaken for evidence that the area was a tidal marine environment in the past. There are many places around the world where the record in the rocks has been interpreted, in samples such  sediment cores, as being tidal marine, but now must be reevaluated.

Sources & Further reading

  1. Ainsworth, R. Bruce, Stephen T. Hasiotis, Kathryn J. Amos, Carmen B.E. Krapf, Tobias H.D. Payenberg, Marianne L. Sandstrom, Boyan K. Vakarelov, and Simon C. Lang. "Tidal Signatures in an Intracratonic Playa Lake." Geology (May 23, 2012 2012).
  2. Pratt, Sara E., October 2012, Tidal Features Found in Lake Sediments, Earth.
Author: M. H. Monroe
Last updated 08/12/2012
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