Mafic and ultramafic rocks of the Valdez and Orca Groups

Unit symbol: Togum
Age range Tertiary, Eocene to Paleocene (66 to 28.1 Ma)
Lithology: Igneous - Plutonic
Group name: Mafic and ultramafic rocks of the Valdez and Orca Groups
Dark-gray, fine- to medium-grained locally porphyritic gabbro that occurs in distinct mafic-ultramafic complexes in the Prince William Sound region. The largest exposure is on Esther Island (Tysdal and Case, 1979). Several small intrusive bodies of gabbro occur on Knight Island, where they intrude sheeted dikes of unit Togb of Nelson and others (1985, included in unit Togum, here). On the Resurrection Peninsula in the Seward quadrangle, Miller (1984) described local occurrences of west-dipping magmatic mineral layering and cumulate textures within the gabbro. The gabbro grades into the sheeted dike map unit (Togv herein) and is generally elongate and parallel to the sheeted dikes, but it also crosscuts the dikes locally (Tysdal and Case, 1979; Miller, 1984). Tysdal and Case (1979) state that, on the Resurrection Peninsula, “The gabbro intrudes slate and sandstone of the Valdez Group, crosscuts the bedding, and forms aphanitic sills in other places. A blue-gray and whitish thermal aureole, at least 200 m wide, marks the contact zone with the sedimentary rocks.” Nelson and others (1985) and Bradley and Miller (2006) instead interpreted the contacts between the gabbro and Valdez metasedimentary rocks on the Resurrection Peninsula as faults. On Knight Island, the gabbro intrudes rocks of the Orca Group. Nelson and others (1989) reported a 57 Ma U/Pb zircon age on a plagiogranite that intrudes the gabbro on the Resurrection Peninsula to be 57 Ma. The ultramafic rocks are primarily exposed in the Cordova quadrangle and in a small exposure associated with the Resurrection Peninsula ophiolite. The larger exposure is part of a compositionally and texturally variable unit that consists mainly of medium-grained gabbro, local diabase, hornblende gabbro, peridotite, and orthopyroxenite (Winkler and Plafker, 1993). Exposures associated the ophiolite consist of dunite, locally with layers of chromite, moderately to mostly altered to serpentine, serpentine-talc, and talc schist (Tysdal and Case, 1979). “In most places enough relict texture and mineralogy remains to recognize original clinopyroxenite, dunite, and harzburgite” (Miller, 1984). Unit occurs as small pods in gabbro and fault-bounded slices within Valdez Group metasedimentary rocks (Miller, 1984; Nelson and others, 1985). On Knight Island, xenoliths of peridotite in sheeted dikes were observed by Richter (1965) but not by subsequent workers (Tysdal and Case, 1979; Nelson and others, 1985). Nelson and others (1985) mapped three peridotite bodies, including two that lie within or near a shear zone in their sheeted dikes unit Tod (unit Togv, here) and the other body occurs as a xenolith in the sheeted dikes unit. These ultramafic rocks weather orange-brown in color and form subdued rubble outcrops

Source map information

Source map Nelson, S.W., Dumoulin, J.A., and Miller, M.L., 1985, Geologic map of the Chugach National Forest: U.S. Geological Survey Miscellaneous Field Studies Map MF-1645B, pamphlet, 16 p., 1 sheet, scale 1:250,000 (Chugach NF Arc/Info coverage 1993).
Symbol Tgd
Unit name Gabbro and diorite
Description Consists of mafic rocks belonging to one of the main intrusive events. These rocks are dominantly medium- to coarse-grained gabbro (labradorite-bearing) and subordinate diorite (andesine-bearing) with finer grained borders of quartz gabbro and quartz diorite. Most of the rocks are subophitic, some are hypidiomorphic- to allotriomorphic-granular and a few are porphyritic. The dominant mafic assemblage is clinopyroxene +/- orthopyroxene. Assemblages of primary + subsequent hornblende and hornblende +/- biotite are also found, mainly in the dioritic and quartz dioritic rocks. Rocks of this unit are considered to be an early mafic phase associated with the granitic plutons of the granite and granodiorite unit (Tg).
Lithology Igneous

Correlated geologic units

Label Toeg
Description Gabbro or gabbro and diorite, Prince William Sound
Geologic age Paleocene to Rupelian
Geologic setting Intrusive, gabbro
Lithology Form Importance
Gabbro < Gabbroic < Plutonic < Igneous Pluton Major
Diorite < Dioritic < Plutonic < Igneous Pluton Minor