EVIDENCE FOR GREENHOUSE GLOBAL CLIMATE: EARLY CAMBRIAN SHADY DOLOMITE, VIRGINIA

A relatively recently exposed road-cut section of the Early Cambrian Shady Dolomite was logged at Porters Crossroads, Virginia. It consists of 300 meters (900 feet) of the late Early Cambrian Shady-Rome succession, and is composed of carbonate parasequences with several distinctive redbed units. The parasequences are 0.3 to 5 meters thick (1-15 feet), and formed on the peritidal portion of a carbonate ramp. The major facies include red mudrock (terrestrial or intertidal mud-flat), red and white sandstone (intertidal sand-flat), microbial laminated dolomite (upper intertidal flat), thick laminated dolomite (lower intertidal), fenestral pellet packstone/mudstone (intertidal to subtidal), and burrowed dolomite/wackestone-mudstone (subtidal). A Fischer (accommodation) plot shows that the succession exposes the late highstand of the Shady carbonate platform, and the transgressive systems tract of the overlying Rome Formation. The parasequences commonly are stacked into groups of 4 to 6, which make up parasequence bundles; this reflects upward thinning of cycles in bundles, which also show progressively shallower water facies upward in a bundle. The roughly 5 to 1 bundling suggests precessional cycles stacked into short term eccentricity cycles. These high frequency fourth-order and fifth-order, upward-shallowing parasequences are classic greenhouse platform deposits, suggesting that the exposed Shady-Rome section formed on a relatively ice-free earth over a time span of at least 2.6 m.y. This study extends the greenhouse climate that typifies the Middle to Late Cambrian back into the Late Early Cambrian.