Insight into the origin of marl-limestone alternation in the Paleozoic offshore epicontinental seas: a case study from the middle Cambrian (~500 My) Marjum Formation, Utah, USA

Middle Cambrian offshore deposits of the Marjum Formation, Utah, USA are characterized by four scales of superimposed cyclicity defined by varying fine siliciclastics versus limestone abundances; these include the limestone-marl couplets (or rhythmites; 5-10 cm) which are bundled into parasequences (1-2 m), and small-scale (5-10 m), and large-scale (20-40 m) sequences. Time series analysis of SiO2 and lithologic rank stratigraphic series reveals cycles consistent with Milankovitch cycles with periods corresponding to Cambrian orbital eccentricity (20 m, 405 ky; ~6 m, 110 ky), obliquity (1.8 m, 30 ky), and climatic precession (1.15 m, ~18 ky). Astronomical calibration of the lithologic rank series indicates that the main sub-Milankovitch cycle at 0.065 m represents ~1 ky and corresponds to the basic rhythmite couplet. All scales of cyclicity are interpreted to be the result of wet vs. dry monsoonal climate oscillations controlling the abundance of fine terrigenous sediment influx to the basin. A plausible millennial-scale climate driver is solar activity. These results provide one of the oldest known geological candidates for solar-influenced climate change modulated by Milankovitch forcing.