VARVES AND VARVE-DERIVED CLIMATE CYCLES? EVIDENCE FROM EOCENE FOSSIL LAKE, GREEN RIVER FORMATION

Lacustrine varves provide an accurate chronologic time scale and aid in establishing depositional rates and climatic history. Laminations of the Eocene Green River Formation in Utah, Colorado, and Wyoming are generally accepted as varves. However, there is evidence that at least some of the laminations from the middle unit of the Fossil Butte Member in southwest Wyoming were not deposited as annual couplets. A study and comparison is made of laminations from basin center to basin margin (~15km) to help elucidate the mode of deposition and determine if perhaps the most distill facies may show evidence of sunspot cyclicity.

A time-synchronous unit, bounded on the top and bottom by 2 cm thick tuffs, was sampled from 5 localities in Fossil Basin. Laminae were counted and studied using a petrographic microscope. Laminae number, thickness, organic content, and grain size were noted. Numeric data from central and marginal localities were analyzed by time-series analysis to detect any significant cyclicity.

At basin margin, unit thickness is 30 cm and laminae number is as great as 1661. Alternatively, basin center is characterized by unit thickness of only 10 cm, and as few as 1238 laminae. Laminae thickness tends to fluctuate dramatically around the mean at the lake margin, but to a lesser degree at the lake center. Spectral analysis has not revealed any cyclicity that can be correlated to the 11-year sunspot cycle. This evidence supports a model of deposition where laminae represent calcite precipitation in response to episodic inflow. This process was likely the controlling factor of deposition for the entire lake, as no sunspot cycles were found in even the most distill samples. It is important that investigators establish that laminae are varves when trying to imply climatically induced depositional cycles. This is especially true in small lakes where inflow processes may be the controlling factor basin wide.