UPWARD SHALLOWING PLATFORM CYCLES: A RESPONSE TO 2.2 BILLION YEARS OF LOW-AMPLITUDE, HIGH-FREQUENCY (MILANKOVITCH BAND) SEA LEVEL OSCILLATIONS

Shallow-water carbonate platforms, characterized by sequences of small-scale upward shallowing cycles, are common in the Phanerozoic and Proterozoic stratigraphic record. Proterozoic small-scale cycles are commonly 1 to 10 m thick, have asymmetrically arranged facies, and are strikingly similar to Phanerozoic platform cycles. In some platform sequences (eg. Rocknest, Wallace, and Helena formations of early to middle Proterozoic age), it can be demonstrated that the lateral distribution of facies within cycles relates to systematic variations in platform paleogeography and topography. In the Rocknest formation, cycles with intervals of tepees and pisolitic breccia formed on a topographic high (shoal complex) near the shelf edge rim, and provide evidence for eustatic falls in sea level at the end of each cycle. The presence of these facies in other Proterozoic cyclic platforms also suggests that eustatic sea level falls may have been important in the development of each cycle. Proterozoic upward shallowing cycles appear to have had periods of between 20,000 and 100,000 years, and probably formed during eustatic oscillations in sea level with amplitudes of less than 10 m. This suggests that cyclicity may have been regulated by Milankovitch band climatic forcing, perhaps influencing global sea level through minor changes related to small-scale continental or alpine glaciation. It is possible, then, that Milankovitch band climatic forcing has occurred for at least the last 2.2 billion years of earth history.