White Clay Creek amphibolite: a piedmont analog of the Catoctin metabasalt

Robert C. Smith, II (retired), and John H. Barnes, Pennsylvania Geological Survey, DCNR, 3240 Schoolhouse Road, Middletown, PA 17057-3534

Analyses of additional samples of the White Clay Creek Amphibolite beyond those studied by Smith and Barnes (1994) in Chester County, Pennsylvania, and adjacent Delaware have extended the known geographic limits of the formation and refined its geochemical affinity. It has now been verified in the Bay View, Coatesville, Kennett Square, Newark West, and West Grove 7 ½' quadrangles. Rather precise correlations are now available from the type locality on White Clay Creek, Pennsylvania, to a principal reference section in the Wilmington and Western Railroad cut, Yorklyn, Delaware. Examination of the relatively large range of compositions, trace element discriminant diagrams, and geologic settings suggests that the White Clay Creek Amphibolite is a continental initial rifting tholeiite. The median composition of the White Clay Creek is similar to the median compositions of the Catoctin Metabasalt of the South Mountain anticlinorium NW of the Tunnel Hill-Jacks Mountain fault system; the Fishing Creek Metabasalt Member of the Sams Creek Formation of Lancaster County; some Sams Creek equivalent metabasalts in southwestern York County; the type and de facto reference section Sams Creek Metabasalt of Maryland; the Accomac Metabasalt of York County; and Catoctin Metadiabase dikes in Grenvillian terranes, such as the Womelsdorf outlier, Reading Prong, Honeybrook Upland, and Trenton Prong. This similarity and limited 143 Nd/ 144 Nd data suggests that all are cogenetic and approximately synchronous. Thus, the White Clay Creek Amphibolite is believed to be of latest Neoproterozoic age as are the others. As such, it should be considered a formation within a mappable continental Laurentian group consisting of it and the host "Wissahickon" and it should make an excellent marker unit to define both the Precambrian-Cambrian boundary and continental (~Humber-)-oceanic (~Dunnage-equivalent) boundary in the Piedmont of southeastern Pennsylvania and adjacent Delaware

. Map showing the distribution of malfic units by population groupings. Click map for larger image(100kb)

Map showing the distribution of mafic units by population groupings. Click map for larger image (100 KB).

This paper was included with the 2004 Field Conference of Pennsylvania Geologists guidebook.