The Talkeetna Formation is dated as Early Jurassic by the presence of the pelecypod Weyla. The species W. unca was reported by Sie Muller (unpublished chart) to range through the Sinemurian into the basal Pliensbachian as identified by ammonites.

Most ammonites collected todate in the Tuxedni Group exposed in the Anchorage D-3 and D-4 quadrangles are of middle and late Bajocian ages. The late Bajocian is represented only by the topmost beds that contain the same ammonites as occur in the Twist Creek Siltstone of the Tuxedni Group north of Cook Inlet between Iniskin Bay and Tuxedni Bay. The middle Bajocian is represented in Anchorage B-3 and B-4 quadrangles by ammonites that occur north of Cook Inlet in Red Glacier Formation and the overlying Fitz Creek Siltstone of the Tuxedni Group. These ammonites furnish a correlation with the Otoites sausei and Stephanoceras humphriesianum zones of Europe which are dated as middle to late middle Bajocian (see Imlay, 1964, Prof. Paper 418-B, p. B7, B14, B18). Still older Bajocian ammonites have been found in the Nelchina area of the Talkeetna Mountains (Prof. Paper 418-B, p. B-14; Prof. Paper 756, p. 33) at USGS Mesozoic locs. 22149, 24235, 25946, 26723, and 29945.

The oldest Bajocian ammonites found todate in the Anchorage D-3 and D-4 quadrangles are from Mesozoic loc. 30600 and probably represent the basal part of the Otoites sauzei zone rather than the highest part of the Sonninia sowerbyi zone. Such a position is indicated by the association of Sonninia, Planammatoceras, Stephanoceras and Witchellia. (Imlay, 1973, Prof. Paper 756, p. 33). The same assemblage was found at Mesozoic 24120 in the Nelchina area (See Prof. Paper 418-B, p. 27).

The next younger Bajocian ammonites are from locality 30582, and are represented by species of Sonninia and Lissoceras that occur in the upper 230 feet of the Red Glacier Formation near Tuxedni Bay and are correlated with the upper part of the Otoites sauzei zone of Europe (Imlay, 1964, Prof. Paper 418, p. B-11, B-12).

The next younger Bajocian ammonites are represented by specimens of Chondroceras allani (McLearn) at Mesozoic loc. 30594 in the Anchorage D-4 quad. It occurs also at Mesozoic loc. 8567 in the Anchorage D-4 quad. (about 3.2 miles up second gulch on north side of Boulder Creek and north of extreme west end of Anthracite Ridge. Collected by Overbeck 8-10-13, notebook I, p. 41, traverse page b). North of Cook Inlet C. allani is common in the upper two-thirds of the Fitz Creek Siltstone as well as in the overlying Cynthia Falls Sandstone (Prof. Paper 418-B, p. B-42, B-43). The beds characterized by Chondroceras allani are correlated with the Stephanoceras humphriesianum zone on the basis of associated fossils (See Prof. Paper 418-B, p. B-12 – B-14; Prof. Paper 756, p. 32, 33). Among such associates is Stemmatoceras cf. S. palliseri (McLearn) which occurs in the Anchorage D-4 quad. At Mesozoic loc. 30591.

Additional age data is furnished by certain species of Inoceramus. Thus, I. Lucifer Eichwald, characterized by an elongate form and fairly smooth sculpture, occurs north of Cook Inlet in the Middle Jurassic beds older than the Fitz Creek Siltstone. I. ambiguous Eichwald, characterized by coarse sculpture, occurs in the Fitz Creek and higher beds.

The beds in southern Alaska that are characterized by Megasphaeroceras, Sphaeroceras, Liroxyites and Leptosphinctes have been assigned an early late Bajocian age for reasons discussed previously (Imlay, Prof. Paper 418A, p. A-2 – A-3, A9-A11; 1967, Prof. Paper 540, p. 26, 27, 28, 29, 59, 61; Prof. Paper 756, p. 31, 33). This age assignment is strengthened by the presence of Stemmatoceras at loc. 30589 (74ADt-106) and possibly at loc. 30583 (74ADt-97).

The ammonites from the top of the Tuxedni Group in the Boulder Creek area are considered to be early Bathonian age by comparisons with the more complete ammonite sequence from the Bowser Formation north of Cook Inlet. Apparently the upper Bathonian, if present, is represented by the 150 feet of unfossiliferous massive volcanic greywacke that underlies the basal conglomerate of the Chinitna Formation.

The stratigraphic positions of the collections made below the volcanic greywacke at the same place in 1972 and 1974 are shown on the following page. Please check and make such adjustments as seem pertinent.

Comparisons of positions of fossil collections made in 1972 and 1974 below 150 feet of volcanic greywacke at the top of the Tuxedni Group exposed on a ridge in NW ¼ SE ¼ sec. 26, T. 22 N., R. 7 E., Anchorage D-3 quad.

Locality 30577 is about at the same position as locality 30275.

Locality 30577 contains an external mold of the same ammonite as collected at locality 30277 and should represent the same unit.

As the fossils form locality 30278 were collected from siltstone about 50 feet below locality 30277, they should occur above the buff-forming greywacke at about the same position as loc. 20578.

The above data shows that the thickness of Cranocephalites-beds is 326 ft. according to measurements by Imlay and 390 ft. according to Detterman. Part of the differences may reflect different determination of amount of dip, or of the thickness of the lower cliff forming unit of greywacke.

The collections made from the Chinitna Formation on the northern tributary of Boulder Creek are probably of middle Callovian age based on the ranges of those genera in Europe. In the Chinitna Formation north of Cook Inlet Stenocadoceras occurs throughout the Paveloff Member and in the upper sixth of the Tonnie Member. The species S. striatum occurs only in the upper two-thirds of the Paveloff Member. Pseudocadoceras grewingki ranges through the upper half of the Tonnie Member and nearly all of the Paveloff Member. The species Cadoceras pomeroyense has been found previously only near the top of the Paveloff Member. Evidently the collections at localities 30585-87 are from beds equivalent to the Paveloff Member. Beds equivalent to the Tonnie Member could be present between locality 30586 and the basal conglomerate exposed nearby one mile further upstream.

Most of the fossils collected from the Naknek Formation in 1974 are poorly preserved and are of no known age significance. The presence of Buchia concentrica from the very base of the formation shows that the formation at that place is of late Oxfordian to early Kimmeridgian age.