The stages of the Lower Jurassic that are represented in the Talkeetna Mountains are based on the European ranges of the ammonites that are present. These ranges and the probable ages of the various fossil collections are listed below. Perhaps with this information you can now determine the ages of the several units of the Talkeetna formation discussed in our recent letter of May 2nd. You will note that most of the collections containing ammonites are of late Toarcian age and that there is no fossil fossil evidence for the Hettangian.

European Ranges of Lower Jurassic Ammonites found in the Talkeetna formation of the Talkeetna Mountains.

Megarietites - Sinemurian, mostly lower
Arnioceras - Sinemurian, most lower
Oxynoticeras - Sinemurian to basal Pliensbachian, mostly upper Sinemurian
Crucilobiceras - upper Sinemurian into basal Pliensbachian
Radstockiceras - lower Pliensbachian
Uptonia - lower Pliensbachian
Acanthopleuroceras - lower Pliensbachian
Protogrammoceras - upper Pliensbachian to lower Turonian
Paltarpites - upper Pliensbachian
Amaltheus - upper Pliensbachian
Pleuroceras - upper Pliensbachian
Catacoeloceras - middle Toarcian
Harpoceras - top of Pliensbachian through Toarcian, but mostly lower & middle Toarcian
Pseudlioceras - middle Toarcian into basal Bajocian (zone of Harpoceras serpentinum to Lioceras opalinum
Phlyseogrammoceras - upper Toarcian
Phymatoceras - upper Toarcian
Haugia - upper Toarcian
Grammoceras upper Toarcian to early Bajocian

Probable ages of Lower Jurassic Fossil Collections from the Talkeetna Mountains

Lower Sinemurian - 24107, 27586
Upper Sinemurian or lower Pliensbachian - 6706, 26722, 24106
Lower Pliensbachian - 3698(?), 6697, 8569, 24108
Upper Pliensbachian - 25941
Lower Toarcian - 24787, 25938(?)
Upper Toarcian - 24109, 24111, 24114, 25316, 25317, 25318, 25319, 25342, 25359, 25939, 25940, 27508, 27509(?)

The basal sandstone of the Tuxedni formation is probably entirely of Bajocian age. In an attempt to determine if any part of it might be of Toarcian age, I examined the fossils from the following localities whiach labeled either as the basal sandstone or from the basal part of the Tuxedni formation.

Fossils from the Basal beds of the Tuxedni formation

[Note by RBB: a list follows of localities with fauna - see locality list]

In summation, the basal sandstone of the Tuxedni formation in the Talkeetna Mountains has furnished ammonites and Inoceramus of early to middle Bajocian age at six localities. The immature ammonite at Mes. loc. 24137 could be either of late Toarcian or early Bajocian age.

I wish that you would let me know if any of the other collections that you have submitted are from the basal sandstone of the Tuxedni formation as I should discuss the age of the basal part of the Tuxedni formation in my forthcoming paper on the Bajocian ammonites.

Also, I would like to have you prepare a very brief statement, or summary of the stratigraphy of the Tuxedni formation in the Talkeetna Mountains that I could use in my paper. You may recall from previous papers that I need a statement concering lithologic features, thicknesses, and stratigraphic positions of the fossils. I could quote your statement as written, or I could use your data in preparing a statement that has some bearing on the fossils. It would be presented as a written communication. I do not need such a statement before next Fall, but would like to have it by December. I am working on the general part of the manuscript now and should complete it by that time. I do not expect the fossil photographs to be available until then, although only about 100 more views are needed.

In summation, the basal sandstone of the Tuxedni formation in the Talkeetna Mountains has furnished ammonites and Inoceramus of early to middle Bajocian age at six localities.

Many of the other collections from the Tuxedni formation in the Talkeetna Mountains that are listed below correlate with the Gaikema sandstone member or with the Red Glacier member as shown by the presence of Inoceramus lucifer Eichwald. This species at Wide Bay occurs as low as the base of the Kialagvik formation associated with ammonites of early Bajocian age. In the Cook Inlet area it ranges to the top of the Gaikema sandstone member and is associated in the upper part of its range (upper part of Red Glacier member and Gaikema sandstone member) with ammonites of early middle Bajocian age. In the Talkeetna Mountains such ammonites occur at Mes. locs. 3696, 3697, 8585, 24113, 24120, 24134, 24149, 24215, 24420, 25345, 25346, 25942, 25945, and 27577. These ammonites include in particular Sonninia, Witchellia, Papilliceras, Bradfordia, Otoites, Emileia, Labyrinthoceras, Skirroceras, Stephanoceras and Parabigotites. These in association are good evidence for correlating the beds in which they occur with the European zone of Otoites sauzei. Otoites and Labyrinthoceras are characteristic of that zone. Papilliceras, Witchellia and Bradfordia are not known above it. Stephanoceras and Emilea occur rarely below it in the upper part of the zone of Sonninia sowerbyi.

Please note that ammonites mentioned above do include Normannites, Chondroceras (except Mes. loc. 8567), Teloceras and Zemistephanus which are common in the Fitz Creek member of the Tuxedni formation at Cook Inlet. Also, note that the first three of these ammonites are characteristic of the European zone of Stephanoceras humprisianum.

Another item to note is that the Bajocian assemblages in the Talkeetna Mountains doe not include any genera typical of the European zone of Sonninia sowerbyi such as Docidoceras, Eumedtoceras, and Euhoploceras. This zone, likewise, has not been identified along Cook Inlet but is well developed at Wide Bay in the Alaskan Peninsula.

In summary the Bajocian ammonites from the Talkeetna Mountains show that lower, middle, and upper parts of the stage are represented but there is no positive evidence for the presence of beds equivalent to the European zones of Sonninia sowerbyi and Stephanoceras humphriesianum. The fossils from Mes. loc. 8567 could represent the last named zone, or the basal part of the upper Bajocian, or could be a mixture from a considerable thickness of beds, but its ammonite assemblage suggests an age slightly younger than that of the Fitz Creek member.

During the next few months I will be compiling distribution charts and maps and writing several chapters concerning the fossils. As these become available, copies will be sent to you and to Bob Detterman for your use and for criticism.

Jurassic ammonites of middle Bajocian age occur in the Tuxedni Group along the northwest side of Cook Inlet and in the Tuxedni Formation in the eastern part of the Talkeetna Mountain, Alaska. Numerically the dominant families are the Otoitidae and Stephanoceratidae. The families Phylloceratidae, Sonninidae, Oppelidae, and Perisphinctidae together compose only about one-third of the total specimens.

The Tuxedni Group has not furnished any ammonite genera that characterize the European zone of Sonninia sowerbyi at the base of the middle Bajocian. It has, however, furnished ammonites that characterize the next higher zones of Otoites sauzei and Stephanoceras humphriesianum.

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