A Basal Cambrian Lava Flow in Northern Virginia

Ancient lavas in Shenandoah National Park near Luray, Virginia

“…but n1ost strike north and clip between 65° E. and 65° \V. Furcron and Woodward (1936) state that the slate "has been found * * * above granodwnte In the absence of any greenstone-a fact that strongly suggests that the extrusive [slate] Is later or younger than either the greenstone or granodwnte"…”

“…The contact between the Catoctin Forn1at10n and the overlying sedimentary beds IS n1arked by a layer of purple or blue slate as much as 150 feet thick, whiCh was mapped by King ( 1950) as the Loudoun FormatiOn In the Elkton area The volcanic nature of this rock IS clear, It contruns numerous amygdules (filled gas bubbles), and In thin sectiOn It shows a relic basaltic fabnc Furcron and Woodward (1936) have descnbed this as an altered rhyolite flow, but the fabric and compositiOn IndiCate that It IS 1nore probably denved from an andesite or basalt Its petrographic character will be discussed In 1nore detail, but Its stratigraphic relatiOns have an In1portant beanng on the relatiOn of the Catoctin FormatiOn to the overlying rocks In the western part of the Shenandoah salient (map In pocket) the greenstone of the Catoctin IS absent, and the purple slate lies directly on granodwnte Still farther west the slate IS absent and the basal beds of the Chilhowee rest directly on faintly foliated coarse-grained granodionte The purple slate IS thus present In several places where the greenstone IS absent Furcron and vVoodward (1936) consider this to be evidence of an angular unconfornuty beneath the Chilhowee and the purple slate As no discordance was noted between the Catoctin and Chilhowee, however, It seems more likely that the relatiOnship IS a result of overlap of the Catoctin Formation onto a topographic high of basement rocks, the purple slate can then be assigned to the Catoctin Formatwn, where Its volcanic character and similanty to other rocks In the volcanic sequence seem to IndiCate that It belongs Along the Skyline Dnve about 4 miles north of Thornton Gap, the slate Is absent, and the basal quartzite of the Chilhowee contains rounded cobbles of fine-grruned massive Catoctin lava, this IndiCates that erosiOn preceded deposi twn of the Chilhowee sedimentary rocks Evidence of a maJor unconformity IS lacking, however Cloos (1951) The structure of the formatiOn IS sinlple North of the Rose River the porphyntic flows and the sechmentary rnembers dip 15° to 20° E or SE The western boundary of the forn1atwn IS the outcrop of the pre-Catoctin eroswn surface and IS Inarked by a few Inches to 150 feet of sediInentary rocks containmg pebbles of the underlymg granochonte On the east the forn1atwn IS cut off by a steeply chpping fault. along which the basen1ent rocks are Intensely <sheared and In places, finely ground to 1nyl01ute There IS a 1 eve1sal of chp of the Catoctin Forn1atwn near this fault At least two n1mor faults cut the fonnatwn In this area One extends fr01n Halvksbill Gap clown Cedar Run, the other goes th1 ough Franklin Cliffs (fig 5) and clown the valley of Rose River…”

Geologic map of the Round Hill quadrangle, Clarke and Loudoun Counties, Virginia, and Jefferson County, West Virginia

“…Beds in central Virginia described by Furcron and Woodward (1936) as a basal Cambrian lava flow unconformable on the Catoctin are probably equivalent to the Loudoun (King, 1950, p. 16) . Reed (1955, p. 892-3), however, has suggested that these beds represent metamorphosed saprolite at the top of the Catoctin metabasalts, and should be included within the Catoctin Formation.…”

“…Furcron and Woodward (1936) interpreted the base of the Loudoun as an angular unconformity. King (1950, p. 17) suggested that the Loudoun lies unconformably on Catoctin and older rocks.…”