Name Rust limestone member proposed for lower member of Cobourg formation in Utica quad and vicinity, NY. Described as argillaceous and coquinal limestone. Slump breccias at Prospect Bridge occur near top. Thickness 115 ft. Underlies newly defined Steuben member; overlies newly defined Russia member of Denmark formation. Age is Middle Ordovician (Mohawkian).

Source: GNU records (USGS DDS-6; Reston GNULEX).

At Trenton Falls, the Rust Member of the Denley Limestone, overlying the Russia Member, consists of 30 m of rubbly fossiliferous wackestones, packstones, and grainstones. Near its base is a previously undescribed metabentonite. Near Rathbun Brook, a few km to the southeast, the Rust, however, is not observed, but is replaced abruptly by the Utica Shale. The change from Dolgeville through Rust and into Steuben lithology marks a significant interval of marine shallowing from dysoxic conditions below storm wave base (Rust) into high energy marine shoal facies (Steuben). Figure 9 illustrates abrupt eastward erosional loss of upper Trenton deposits near Poland and progressive westward overlap of Utica deposits over progressively younger Trenton units. Figure 10 offers an alternative interpretation for the disappearance of the Rust involving a submarine fault scarp and a graben depocenter. Work on the problem is continuing.

Source: GNU records (USGS DDS-6; Reston GNULEX).