Building a robust Middle Cambrian astrochronology using deep-water rhythmites and parsequences in the Marjum Formation of western North America

Abstract

<p>During the Middle Cambrian, western Utah (USA) occupied the deepest part of the House Range embayment along the western edge of North America and accumulated deep-water facies dominated by cm-scale, cyclically interbedded limestone-marl layers (‘rhythmites’) of the Marjum Formation. The studied section is ~500 m thick and spans the lower Drumian-lower Guzhangian. The rhythmites (cm-scale) are packaged into repeated 2-5 m-thick parasequences and larger scale eustatic sequences both characterized by more carbonate-dominated rhythmites alternating with more siliciclastic-dominated rhythmites. Previous studies of the Marjum rhythmites suggest millennial-scale interpretation for the cm-scale rhythmites, corresponding to wetter vs drier climate oscillations influencing the relative abundance of siliciclastic vs carbonate accumulation in the deep-water setting (Elrick and Hinnov, 1996; Elrick and Hinnov, 2007).</p><p>We conducted a high-resolution study of a representative 35 m-thick interval of the Marjum Formation to build a floating astrochronology assuming the meter-scale parasequences represent orbital-scale frequencies modulated by the larger scale sequences (based on biostratigraphically controlled sediment accumulation rates). To capture orbital-scale frequencies and to avoid aliasing related to millennial-scale limestone-marl lithological changes, 350 rock samples were collected at minimum 10-cm-interval exclusively in the limestone layers of rhythmite couplets. Major elements (e.g., Ti, Al, K, Si), TOC and δ<sup>13</sup>C were measured on each sample to infer on paleoenvironmental and paleoclimatic conditions. To estimate the duration of the sampled interval, multiple spectral techniques will be applied on the paleoclimatic proxies using amplitude modulation, frequency ratio as well as the 405 kyr long eccentricity as a starting point for the calibration of the time scale. Having an astronomically calibrated time scale for this interval will provide 1) independent evidence assessing the millennial-scale paleoclimatic origin of the cm-scale rhythmites, and 2) refine the Cambrian time scale to better constraint the origin and rate of the Cambrian explosion events.</p><p>Elrick, M. and Hinnov, L.A., 1996. Millennial-scale climate origins for stratification in Cambrian and Devonian deep-water rhythmites, western USA. Palaeogeography, Palaeoclimatology, Palaeoecology, 123(1-4): 353-372.</p><p>Elrick, M. and Hinnov, L.A., 2007. Millennial-scale paleoclimate cycles recorded in widespread Palaeozoic deeper water rhythmites of North America. Palaeogeography, Palaeoclimatology, Palaeoecology, 243(3-4): 348-372.</p>