Collection from Nasorak Creek sea-cliff section. (from faunal list given on Table 2, pp. 8-9 in Campbell, 1967)

The name Kogruk(?) Formation is applied to thick dolomitic section (unit Ml₄ of Campbell 1960a, b) in the Lisburne Group of this area because its stratigraphic position is similar to that of the Kogruk Formation as described by Sable and Dutro (1961, p. 592) in its type area in the western DeLong Mountains (from Campbell, 1967, p. 14)

Fossil material consists of abundant crinoid columnals and undetermined echinoderm debris and subordinate Bryozoa, horn corals, small colonial corals (including lithostrotionoid corals, particularly near the base), brachiopods of several species, and at least one blastoid (table 2). The fossil collections were examined by J.T. Dutro, Jr., and Helen M. Duncan, of the Geological Survey, who report (written commun., 1961) that the fossils appear to correlate with those in the upper part of the Alapah Limestone of the central Brooks Range. Several collections represent a Chester-type assemblage of bryozoans, brachiopods, and echinoderms. Dutro and Duncan also note that the collections correlate broadly with the Gigantoproductus zone of the central Brooks Range and that Gigantoproductus has been found in similar rocks near Cape Lisburne. They further suggest (written commun., 1961) that the Kogruk(?) Formation and most of the underlying Nasorak Formation are faunally equivalent to the type Kogruk of the western DeLong Mountains. The presence of Late Mississippian fossils in the Kogruk(?) and in the upper part of the underlying Nasorak Formation suggest that all the rocks assigned to the Kogruk(?) in this area are of Late Mississippian age, whereas the type Kogruk Formation of the western DeLong Mountains is considered to be of Early and Late Mississippian age (Sable and Dutro, 1961, p. 592). (from Campbell, 1967, p. 18)

(Referring to unit 1 of the upper member of the Nasorak Formation Kogruk on p. 44 of Campbell, 1967): Fossils include colonial corals and brachiopods as well as echinoderm and bryozoan debris. Colonial coral heads locally stand out as white lenses as much as 1 ft thick and 1 1/2 ft long in the limestone beds (fig. 7). 59ACr-22f, 59ACr-23f, and 59ACr-24f (table 2) are fossil collections.

They can be correlated in a broad way with the Mississippian sequences in the central and western Brooks Ranges. The collections are discussed from the top down.

Unit Ml₄ appears to correlate with the upper part of the Alapah limestone of the central Brooks Range. Because most of unit Ml₄ represents a different facies from the type Alapah, the faunas have a different aspect. Several collections represent a "Chester"-type assemblage of bryozoans, brachiopods, and echinoderms. Collection 55 best represents this assemblage, with Archimedes and Pentremites and several kinds of brachiopods.

In a broad way, these collections correlate with the Gigantoproductus zone of the central Brooks Range. Gigantoproductus has been found in similar rocks near Cape Lisburne, north of the Point Hope area, although none appear in this shipment.

Collections above 56 are poorly preserved in a dolomitic matrix and cannot be definitive in terms of age or correlation.

Unit Ml₃, except for the basal part, is probably equivalent to the lower part of the Alapah limestone. There is a wide variety of lithostrotionoid corals, like those found in the Lithostrotion aff. L. asiaticum (Yabe and Hayasaka) through Sciophyllum lambarti (Harker and McLaren) zones of the central Brooks Range sequence.

The basal part of unit Ml₃, including collections 11, 12, 13, 123 and 122, is probably correlative with the upper part of the Wachsmuth limestone, the Brachythyris suborbicularis (Hall) zone.

The single collection from Ml₂ contains no definitive fossils but since this unit is bracketed by Wachsmuth age collections, it too can be be considered a correlative of the upper Wachsmuth.

Collection 119, from unit Ml₁, also represents the upper part of the Wachsmuth and is similar to collection 123. Therefore, units Ml₁, Ml₂ and the lower part of Ml₃ can be considered of Osage age.

Unit Ml₁, and the upper part of the unnamed clastic unit below, I would consider equivalent to the Utukok Formation of the western De Long Mountains. Units Ml₂, Ml₃ and probably Ml₄ would be equivalent to the Kogruk Formation. Unit Ml₅ is a Tupik equivalent. I suspect that repetition by faulting may have resulted in too thick a measurement for unit Ml₄ and too thin a measurement for Ml₅. In 1951, we measured at least 490 feet of the Ml₅, Tupik equivalent. From the measured sequence, as transmitted with the examination request, I would suggest that only about 2400 feet is certainly assignable to Ml₄. The upper part may be repetitions of Ml₄ and Ml₅. In addition, there is evidence of intertonguing of the Ml₄ and Ml₅ facies, not only in the Point Hope area but also in the central Brooks Range. This usually complicates any cut-and-dried formations in the upper part of Alapah equivalents.