Tidal Rhythmites and Their Implications in Surmising Earth-Moon
Dynamics
Coughenour, Christopher Lynn1
(1)
Tidal rhythmites
are deposits of alternating coarse and fine sediments with foreset
or laminae thicknesses that vary rhythmically and
preserve tidal periodicities. Quantitative analyses of tidal rhythmites have improved and resulted in more precise
elucidation of tidal periodicities encoded in the rock record. The resulting
reconstruction of the history of lunar retreat from these data indicates that
the earth is currently experiencing a high rate of tidal dissipation. The
paucity of long-term, high resolution deposits, however, has allowed only broad
trends in the history of the Earth-Moon system to be resolved. The use of more
common, time-abbreviated rhythmites representing only
months of deposition is likely necessary for more precise interpretations.
There is evidence that suggests Earth's
moment of inertia and the total angular momentum of the Earth-Moon system have
changed little through time. By assuming a perfectly spherical Earth and a Moon
in circular orbit, the equations for angular momentum are simplified. The
errors introduced via these simplifications are relatively small. The resultant
equations for angular momentum are combined and it is assumed that the total
angular momentum of the Earth-Moon couplet is conserved. Preliminary comparison
to a 60-year continuum of sediments yielding precise values for the sidereal
month and Earth-Moon distance suggest that the assumptions made by this method
are accurate to within 10%. The results seem to affirm similar assumptions made
in numerical models of lunar retreat, perhaps, providing a means of validating
such models.
AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California