McHugh and Uyak Complexes

Unit symbol: Kumc
Age range Late Cretaceous (89.8 to 72.1 Ma)
Lithology: Tectonite
Group name: McHugh and Uyak Complexes and similar rocks
Tectonic mélange that consists of fault-bounded blocks that have protolith ages of Mississippian through mid-Cretaceous; largely of oceanic affinity (Clark, 1973; Connelly, 1978; Tysdal and Case, 1979; Winkler, 1992). Unit is found inboard of the Valdez Group and Kodiak Formation (unit Kaf) around the Gulf of Alaska. According to Clark (1973), the McHugh Complex consists of two lithologically distinct but structurally juxtaposed packages. Dominant or most common is a metaclastic sequence “composed predominantly of gray, gray-green, and dark-green weakly metamorphosed * * * siltstone, graywacke, arkose, and conglomeratic sandstone” (Clark, 1973). A metavolcanic sequence consists of “greenstones of mostly basaltic composition and texture that are commonly associated with radiolarian metachert, cherty argillite, and argillite. Small amounts of ultramafic rocks and marble occur locally as isolated, discontinuous outcrops or lenticular masses” (Clark, 1973). Uyak Complex of Connelly (1978) is similar in character, but the metasedimentary (metaclastic-equivalent) part consists dominantly of deformed gray chert and argillite (Connelly, 1978). Within the McHugh and Uyak Complexes, “broad zones as wide as 1 km of intense shearing lack any stratal continuity and, in many places, are marked by angular, elongate phacoids, either enclosed in pervasively sheared matrix or juxtaposed against other phacoids. Larger phacoids are lithologically diverse, consisting of schist, amphibolite, marble, sandstone, conglomerate, diorite, gabbro, serpentinized ultramafic rocks, and mafic volcanic rocks” (Winkler, 1992). Metamorphic minerals include muscovite, epidote, calcite, chlorite, albite, and veinlets of prehnite (Tysdal and Case, 1979). Blocks of mafic and ultramafic rocks are serpentinized near their margins (Connelly, 1978). Slickensides are common both as subparallel anastomosing fractures in competent rocks and as closely spaced fractures in less competent rocks Connelly (1978). In the McHugh Complex, sedimentary rocks of the matrix have yielded Early Cretaceous (Valanginian) fossils, whereas protolith ages on blocks in the mélange have yielded radiolarians of Cretaceous (Albian-Aptian), Jurassic, and Triassic age (Bradley and others, 1999; Winkler and others, 1981), Permian conodonts and fusulinids of Tethyan affinities (Connelly, 1978; Stevens and others, 1997; Bradley and others, 1999), and Mississippian to Pennsylvanian conodonts (Nelson and others, 1986). Bradley and others (2009) reported 90-Ma detrital zircons from metasandstone collected along Turnagain Arm in the Anchorage quadrangle, within the McHugh Complex. Bradley and others (1999) locally subdivided the McHugh Complex into distinct lithologic packages loosely similar to the two packages originally defined by Clark (1973)

Source map information

Source map Bradley, D.C., Kusky, T.M., Haeussler, P.J., Karl, S.M., and Donley, D.T., 1999, Geology of the Seldovia quadrangle: U.S. Geological Survey Open-File Report 99-18, 1 sheet, scale 1:250,000.
Symbol KPms
Unit name McHugh Complex, graywacke and conglomerate
Description Fault-bounded slices of massive conglomerate and graywacke.
Lithology Sedimentary

Correlated geologic units

Label KJms
Description McHugh Complex, graywacke and conglomerate
Geologic age Pliensbachian to Early-Cretaceous
Geologic setting Melange
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Melange Major
Graywacke < Sandstone < Clastic < Sedimentary Melange Major