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Field observations of Early Jurassic Watchung Basalt throughout the Watchung syncline, including 17 large trap-rock quarries, provide new data on the number of flows, their structure, and the eruptive style of the lavas. Three Orange Mountain Basalt flows, five Preakness Basalt flows, and two Hook Mountain Basalt flows are mapped.

The volcanic structures and the eruptive style of the Watchung Basalt flows are typical of continental flood-basalt provinces. The great thickness and large lateral extent of Watchung flows and their pahoehoe surfaces confirm geochemical evidence that viscosity of the lavas was low and eruptive rates were high. Low viscosity of the First Preakness flow and its unusually great thickness (up to 150 m) may account for the thick, phaneritic, diabasic zones and the small lenses of basaltic pegmatite in the flow.

Several different kinds of flow structures, including cooling joint patterns and vesicle distributions, indicate that most Watchung lava flowed onto a dry land surface but that some flowed into large lakes. Subaerial cooling of most Watchung flows produced a shrinkage joint pattern that consists of a single basal colonnade and a well-developed entablature. These early primary shrinkage joints were modified later by tectonic joints. The Watchung Basalt is cut by strike-slip faults that strike N30°W. These faults are consistent with north-south compression that coincided with development of the Watchung syncline.

Chemical alteration processes that have modified the Watchung Basalt to varying degrees include weathering, salt-water reactions, sediment assimilation, hydrothermal activity, and metasomatism. The lower colonnades of most flows have been particularly affected by saline waters, resulting in major variations in SiO2, FeO, MgO, CaO, Na2O, K2O, Cu, and Zn.

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