ABSTRACT Two construction aggregate companies, Cemex and Hanson Aggregates, operate respective crushed stone quarries on the east and west slopes of Mount Zion in Clayton, California. These sidehill quarries utilize a single highwall and mine Jurassic diabase of the Coast Range ophiolite that formed as a sheeted dike complex. Hydrothermal veins, some containing 20%–30% disseminated pyrite and chalcopyrite, cut the diabase. The east quarry, operated by Cemex, was started by the Harrison-Birdwell Company in 1947. The west quarry, operated by Hanson, was started by the Henry J. Kaiser Sand and Gravel Company in 1954. The Cemex quarry highwall is visible as you come into the city of Clayton on Marsh Creek Road, with a height of ~280 m (920 ft). The height of the highwall at the Hanson quarry is ~215 m (700 ft). Both operations remove weathered diabase overburden to expose fresh diabase, which is drilled, blasted, and hauled to the plant for processing. To ensure aggregate is suitable for construction, quality assurance testing is conducted in accordance with the specifications of various agencies. These quarries supply the surrounding area with aggregate for hospitals, schools, highways, dams, and other buildings. Noteworthy projects supplied by the Clayton quarries include the Concord BART Station, Interstate-680, Interstate-580, Calaveras Dam, Sherman Island Levee, Highway 4, Highway 24, and Bay Bridge epoxy asphalt. Before aggregate was mined, Mount Zion was the site of a copper rush from 1862 to 1864. Gold and silver were also reported in various assays from the Clayton district. Although prospecting created excitement around Clayton, no productive orebodies were ever discovered.

Detailed petrographic and geochemical analyses of samples collected from eight cores of the U.S. Army Corps of Engineers Passaic Tunnel project indicate that the three principal basalt formations of the early Mesozoic Newark basin in New Jersey are composed of multiple flow units that can be identified on the basis of physical and petrochemical criteria. The high-titanium, quartz-normative (HTQ) Orange Mountain Basalt (lowermost formation: at least three flow units) and high-iron and high-titanium, quartz-normative (HFTQ) Hook Mountain Basalt (uppermost formation: at least two flow units) include flows characterized by a limited range in composition and are indicative of eruptive cycles during which individual magma sources underwent little change in composition and were tapped repeatedly. In contrast, the wide chemical diversity of the flows in the Preakness Basalt (middle formation: possibly nine flow units) is indicative of a period of volcanism in which successive eruptions were derived either from (1) separate high-iron, quartz-normative (HFQ) and low-titanium, quartz-normative (LTQ) parental magmas; or (2) an LTQ parent that changed composition during the eruptive interval. The HTQ composition and limited variability of the Orange Mountain Basalt indicate petrochemical correlation with the stratigraphically comparable Mount Zion Church (Culpeper basin, Virginia) and Talcott (Hartford basin, Connecticut) Basalts. The wide range in composition of the Preakness Basalt supports previous petrochemical correlation with the Hickory Grove and Sander Basalts of the Culpeper basin. The HFQ compositional type of the Preakness, Hickory Grove, and Sander Basalts is similar to the Holyoke Basalt of the Hartford basin (Connecticut and Massachusetts) and Deerfield Basalt of the Deerfield basin (Massachusetts). The HFTQ Hook Mountain Basalt is nearly identical to the stratigraphically comparable Hampden Basalt of the Hartford basin (Connecticut and Massachusetts) in both composition and limited degree of compositional variability. These new data and conclusions drawn from recent cyclostratigraphic investigations of the intercalated sedimentary strata from these basins define three nearly synchronous eruptive intervals for the Eastern North America (ENA) province from Virginia to Massachusetts. Volcanic interval I, which followed an extended period of sedimentation in the Culpeper, Newark, and Hartford basins, was marked by repeated production of exclusively HTQ-type basalt and involved parental magmas that did not significantly change composition during the eruptive episode. Volcanic interval II, which followed a second, shorter period of basin sedimentation occurring throughout the province, began with eruption of the Hickory Grove Basalt in the Culpeper basin and, following a period of renewed terrestrial sedimentation in this basin, resumed with nearly simultaneous extrusion of HFQ-type basalt in all of the basins. In the northern basins (Hartford and Deerfield), where basalt eruption was uninterrupted by significant sedimentation, this interval involved only HFQ-type basalt. However, in the southern basins (Culpeper and Newark), where basalt production was interspersed with one or more periods of sedimentation, lavas defined a range of compositions, including LTQ-type and other hybrid varieties. Volcanic interval III, which followed another significant period of sedimentation in the Newark and Hartford basins, involved production of exclusively HFTQ-type basalt from parental sources that underwent slight change in composition during the episode. The similarity of inferred parental magma compositions from Virginia to Massachusetts implies the effect of large-scale petrologic control on melt production. The time-composition relationships of the basalts indicate that the volcanic sequences of the ENA province were produced by episodic tapping of compositionally distinct parental sources that varied in the extent of volcanic production on both a regional and intrabasin scale.

A major carbonate sequence occurs within the lower part of the Talladega slate belt in Chilton, Coosa, and Talladega Counties. The carbonate units are overlain by a major regional unconformity known as the pre-Lay Dam Formation unconformity. The carbonate sequence below the unconformity is represented in different areas by the Jumbo Dolomite, the Marble Valley carbonates, and the Sylacauga marbles. At the type location in Chilton County, the Jumbo is a 67-m-thick, predominantly thickly bedded dolostone. The contact with the underlying slates of the Wash Creek Slate (Mount Zion Formation) is an interlayered zone of dolostone and fine, commonly graphitic, clastic rock. This zone grades upward into a dolostone that contains a few pelitic layers. Near the base, the Jumbo contains intraclasts and recrystallized fragments up to 6 cm in length. Rounded quartz grains are disseminated in the lower section of the Jumbo. Near the middle of the massively bedded dolostone is a layer that contains intraclasts of massive and laminated carbonate as much as 12 cm in length. Just below the unconformity in the type section, the upper part of the Jumbo contains laminations of fine-grained clastic rock. Along strike to the northeast and southwest the unconformity appears to have erosional relief. Less than 1 km west of the type location the unconformity truncates the carbonate sequence completely. To the northeast the pre-Lay Dam Formation unconformity appears to rise in the section in the Marble Valley carbonates and the Sylacauga marbles, exposing a very thick carbonate sequence. Stratigraphic and structural relationships of the Jumbo, Marble Valley, and Sylacauga marbles are not yet resolved. Initial data indicate that the Jumbo occurs stratigraphically below and to the northwest of the Marble Valley carbonates, suggesting that the Jumbo is the oldest carbonate unit in the sequence.

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