Quantitative Analysis of a Cyclic Peritidal Carbonate Sequence, the Middle and Upper Devonian Lost Burro Formation, Death Valley, California--A Possible Record of Milankovitch Climatic Cycles

Abstract

ABSTRACT The middle and upper part of the type section of the lost Burro Formation is composed of 186 m of peritidal limestone, dolomite, and quartz arenite. Meter-scale analysis of lithology, sedimentary structures, fossil assemblages, and microfacies delineated 54 primary depositional cycles of subtidal, intertidal, and supratidal facies and a larger-scale trend of apparent sea-level change. Gamma analysis of Kominz and Bond (1990) tests the assumptions that cycles have equal durations and that each facies has a unique time-thickness relation. In the primary gamma and spectral analysis, signals of Milankovitch climatic cycles are indicated on the spectrum of the spatial facies curve, but gamma tuning failed to improve the spectrum. In subsequent analysis, the 54 cycles are subdivided into fi e subgroups of different mean durations based on cycle period distributions. Gammas of three subgroups are strikingly constant, indicating that they least violate the gamma assumptions. The grouping of cycles suggests that the primary cycles do not have the same duration. Gammas are used to convert the measured section into two time series: one is gamma uncorrected (uniformly stretched), another gamma corrected (nonuniformly stretched). After calibrating the gamma time unit as 18.4 ky, eight statistically significant spectral peaks on the gamma-corrected spectrum match the predicted 377 Ma Milankovitch eccentricity (2035, 413, 131, and 95 ky), obliquity (32 ky), and precession index (20, 17, and 9 ky (the constructive tone)) peaks. The match suggests that Milankovitch climatic forcing wa an important control on the Lost Burro carbonate deposition. The effective accumulation rates of the subtidal (280 mm/ky), intertidal (160-190 mm/ky), and supratidal (60-70 mm/ky) facies are used to establish the cyclostratigraphy of the measured section.