DETRITAL ZIRCON AGES FOR THE CAMBRIAN MONKTON AND DANBY FORMATIONS, CHAMPLAIN VALLEY, VERMONT

The Monkton and Danby Formations of the Cambrian shelf stratigraphic sequence in Western Vermont (VT) are comprised of arkose to sub-arkose sandstones and dolomite. Biostratigraphic relationships of the Monkton Formation to the Potsdam Group in New York (NY) by previous workers suggest they would be at least partially correlative. Detrital zircon studies have been completed on the Potsdam Group by others to identify to help constrain the age and provenance of this stratigraphy. This study was done to complete the first detrital zircon study of Cambrian stratigraphy in the Champlain Valley of Vermont (VT) to better constrain the provenance of this stratigraphy and to better correlate it with the Cambrian strata of NY.

Samples were collected at Redstone Quarry in Burlington, VT and Shelburne Falls in Shelburne, VT for the Monkton and Danby, respectfully. Samples were analyzed by laser ablation–inductively coupled plasma mass spectrometry (LA-ICPMS) at University of Arizona Laserchron Center. Backscattered Electron (BSE) imaging for both samples was done to identify zircons for analysis within the mounts. LA-ICPMS analysis of 279 spots for the Monkton and 298 for the Danby was completed. Cathodoluminescence (CL) imaging of the two mounts was completed at Middlebury College to identify the locations of the spots within the zircon and the two populations morphologies.

Zircon population distribution shows two dominate age peaks between 1.05-1.09 Ga and 1.15-1.18 Ga for the Monkton and Danby suggesting either a continuity of provenance source through the Cambrian or the cycling of the Monkton’s sand. The 1.05-1.09 Ga age range corresponds to the Ottawan Orogeny while the 1.05-1.09 Ga range to the Shawinigan Orogeny and AMCG plutonism. Dominant age peaks in the VT samples between 1.15-1.18 Ga are similar to the 1.16 Ga age peak seen in the Altona and Ausable Formations of the Potsdam Group of NY. A local (VT) Grenville source for the zircons is rejected based on paleogeographic reconstruction of the Iapetian shelf margin. The complex age distributions described by others in the younger members of the Potsdam Group are not statistically significant in the Vermont stratigraphy.