RE-ASSESSING RELIABILITY OF METER-SCALE CYCLIC CARBONATES AS RECORDERS OF ORBITAL EVENTS: MIDDLE CAMBRIAN HIGHLAND PEAK FORMATION, SOUTHERN NEVADA

Gamma analysis of a 150-m-thick section in Member 9 of the Highland Peak Formation was used to test whether or not orbital signals are recorded consistently over a relatively short distance on a platform. The Highland Peak Formation represents over 1000 m of shallow-water carbonates deposited on a broad carbonate platform along the passive margin of western Laurentia during the Middle Cambrian. Member 9 correlates to the Trippe Limestone in south-central Utah, where the gamma method coupled with spectral analysis successfully revealed orbital variations with Milankovitch bandwidth frequencies (Bond et al., 1991).

Theoretically, gamma analysis establishes an effective accumulation rate in cyclic carbonates whose ages are poorly constrained. This artificial rate attempts to compensate for changes in rates of sedimentation, erosion, non-deposition and diagenesis such that orbital periodicity recorded in sedimentary rocks can be deciphered more accurately. Requirements for gamma analysis include a continuous section containing approximately 20 to 40 cycles. A cycle is composed of a succession of facies beginning with deposition of deepest water facies and ending with deposition of shallowest water facies.

Member 9 comprises 30, approximately 5-m-thick, shallowing-upward cycles. Three dominant facies are (A) burrow-mottled mudstone and wackestone with occasional stromatolitic boundstone, (B) intraclastic grainstone and packstone, (C) cryptmicrobial laminite boundstone with rare thin beds of oolitic/peloidal/intraclastic grainstone. Gamma analysis of these cyclic carbonates could not produce a viable effective accumulation rate, indicating that these meter-scale shallowing-upward cycles either had widely varying sediment accumulation rates and/or widely varying cycle durations. Spectral analysis, consequently, does not resolve true Milankovitch frequencies, which are masked by sedimentological and post-depositional processes. Thus, temporally correlative cyclic carbonate successions in close physical proximity and in similar depositional environments on the same platform may not consistently record periodicity that can be inferred as orbital events.