Weigand and Ragland (1970) have demonstrated that most eastern North American Mesozoic tholeiites may be classified on the basis of their chemical composition into at least four distinct populations: (1) olivine normative, (2) high-TiO2 quartz normative, (3) low-TiO2 quartz normative, and (4) high-Fe2O3 quartz normative. The determination of just where the Watchung Basalt fits into this classification scheme, together with its genetic implications, is a major objective of this paper. We also describe how the structural changes that occurred during the initial stages in the development of the Atlantic Ocean spreading center may have influenced the chemical compositions of the three Watchung Basalt units.
The Watchung Basalt units (Fig. 1), together with mudstones interbedded between them, dip to the northwest at about 15° (Darton, 1889). The interbedded mudstones are part of the upper Brunswick Formation, which is the uppermost of three formations that together make up the Newark Group (Kummel, 1897). The lower and middle units are the Stockton and Lockatong Formations. The basalts are exposed in central New Jersey as three long ridges separated by valleys cut into the relatively soft mudstones. Stratigraphically from bottom to top, the three ridges are known as the First, Second, and Third Watchung Mountains and average 183, 229, and 91 m thick, respectively (Faust, 1975). A fourth basalt ridge located west of the Third Watchung was recognized by Black and Piburn (1972) as chemically equivalent to the Third Watchung and has been interpreted by them as Third Watchung re-exposed as part of the west limb of a fold. The flows are in each case the result of fissure eruptions (Faust, 1975).