At the base of the Cenozoic rock sequence in the southern Egan Range are Eocene lacustrine carbonate and clastic rocks, in places more than 3000 feet thick, deposited in a structural and/or physiographic basin. These are overlain unconformably by Oligocene and Miocene(?) conglomerate, tuffaceous sedimentary rocks, welded tuffs, and volcanic flows which filled the early Tertiary depressions, accumulated near vents, and, by means of glowing avalanche deposits, finally spread over much of the land surface. The welded tuffs grade upward into Miocene and Pliocene(?) fluviatile and tuffaceous beds. Latest Tertiary (late? Pliocene) and Quaternary rocks include basalt flows and alluvial beds deposited with angular unconformity upon older Tertiary and Paleozoic formations.

Prior to deposition of the Eocene lacustrine rocks at least part of the area had been broken by near-vertical reverse faults and by a large east- and northeast-trending normal fault. An erosional topography of moderate relief had developed and large blocks of Upper Paleozoic rocks had slid into the incipient lake basins. Renewed structural activity during Oligocene and Miocene time, the initial phase again marked by the emplacement of large slide blocks, resulted in great displacement on the large normal fault and on newly developed moderately dipping (31°–56°) normal faults. Displacement of about 3.9 miles, including a large component of strike-slip, occurred on the largest of the newly developed normal faults.

During Pliocene time the present Basin and Range fault pattern was superimposed upon the older structural system, the rocks were tilted an average of 22° to the east-southeast (making low-angle normal faults of the moderately dipping Oligocene-Miocene faults), and the ranges were formed. Displacement on the main Basin and Range faults has continued until quite recently as shown by the eastward tilt (average of 9°) of uppermost (?) Pliocene and Pleistocene beds and by the presence of a fault in the Recent alluvium along the west front of the Egan Range.

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