THE GREAT PALEOZOIC UNCONFORMITY: PROVENANCE OF NEOPROTEROZOIC-CAMBRIAN ROCKS OF THE APPALACHIANS USING DETRITAL ZIRCONS

The base of the Paleozoic section in North America is a great unconformity. In the Appalachians from VT to PA, Early Cambrian clastic rocks assigned to six formations unconformably overlie Mesoproterozoic metagranitoids (Yg). In the allochthonous Blue Ridge (BR) from PA to GA, Early Cambrian Chilhowee Group (Cch) rocks assigned to 8 formations unconformably overlie Yg and 4 Late Neoproterozoic units. In the Adirondacks, NY, and Llano Uplift, TX, Late Cambrian clastic rocks unconformably overlie Yg and Paleoproterozoic rhyolites and granitoids in the Mid-Continent, MO. Cambrian rocks were deposited as part of a fluvial-to-marine transition on the newly formed Neoproterozoic Laurentian passive margin.

Twenty samples of Cch rocks were collected from a broad, telescoped section, occurring in 15 different Paleozoic thrust sheets of varying displacement. Samples were obtained from the west and east limbs and core of the BR, and 3 tectonic windows. To assess changing provenance with time, detrital zircons (DZ) from 5 pre-unconformity units and 4 post-unconformity units were analyzed. In total, about 2,700 DZ from 36 samples were dated by LA-MC-ICPMS. 207Pb/206Pb ages were used for constructing Relative Probability curves.

Two major age populations of 1170 and 1050 Ma are consistent with published U-Pb SHRIMP crystallization ages of Grenvillian Yg units from VT to GA. In addition, 18 samples yielded a few DZ with ages of 3.1-1.35 Ga, reflecting additional sources from the Superior Province (>2.5 Ga), Central Plains (1.8-1.6 Ga), and Mid-Continent (1.5-1.35 Ga) regions. DZ from two basal sandstones of Late Cambrian age (in MO and NY) yield ages that are consistent with the underlying crystalline rocks (1.8-1.3 and 1.2-1.0 Ga, respectively).

Early Neoproterozoic sediments contain locally derived DZ reflecting rapid exhumation of the orogen. Post-Grenvillian zircon sources include local plutonic and volcanic rocks. DZ ages of 788±26 to 540±22 Ma extend the range of ages of rocks exposed and dated in the BR, but these rocks may be buried beneath the Paleozoic allochthons.

Thus, our new age data indicating derivation of basal Cambrian clastic rocks of eastern North America from underlying basement support the concept of a continental-scale unconformity in the early Paleozoic.