Jurassic intercalated micrite and quartz silt lagoonal laminae: Cyclostratigraphic deposition indicative of Schwabe 11‐year solar cycles

ABSTRACT A 4 m thick deposit of intercalated thin micrite and silt‐sized quartz couplets is present within the Middle Jurassic Hulett Member of the Sundance Formation in Wyoming, USA. These deposits accumulated in a hyperarid restricted lagoon. The thicknesses of over 1300 couplets were measured, and each couplet has been interpreted as representing an annual deposit, i.e. a varve. Fast Fourier transform analyses identified an 11‐year periodicity in thicknesses of the laminations. This 11‐year occurrence of thicker laminae coincides with the pattern of the 11‐year Schwabe sunspot cycles. A minor number of deviations from this pattern were noted, possibly resulting from factors such as local climate variability, erosion and non‐cyclical or episodic depositional events. Based on the measurements and numbers of laminae couplets, it took marginally more than 7000 years for the infilling of the Jurassic lagoon, indicating a fairly rapid rate of accumulation. The findings demonstrate the significant impact of solar cycles on depositional periodicity and their influence on these thinly laminated strata, i.e. accumulation of a cyclostratigraphic depositional pattern.