In the Early Silurian a broad subtidal to peritidal shallow carbonate ramp was situated in central and northern Nevada. Along both the northern and western margins the shallow ramp merged into a deep-water basin. The northern shallow-ramp margin, coincident with the Tooele Arch, had a narrow grainstone shoal facies belt that passed downslope into deep-ramp environments. The grainstone shoal facies at the western shallow-ramp margin formed a wide belt to the north and a narrow belt to the south. Shallow-ramp sediments thinned northward toward the shallow-ramp margin, indicating slight differential down-to-south subsidence of the shallow ramp.

Later in the Silurian (C4—Llandovery) the ramp was abruptly drowned. Simultaneous down-to-north subsidence along the northern shallow-ramp margin possibly led to surface faulting and the formation of an escarpment-bounded shelf.

Subsequently, the drowned ramp evolved into a rimmed shelf, the rim consisting of high-energy skeletal sand shoals. Eastward progradation of peritidal sediments from the shelf rim filled in the shelf during the Llandovery and Wenlock. The rim facies also prograded oceanward over the western slope during the later Silurian. Along the northern shelf margin the rim facies was less well developed, and oceanward progradation over the northern slope was slow.

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