Early and middle Bajocian (Middle Jurassic) ammonites from southern Alaska

Abstract

Early and middle Bajocian (Middle Jurassic) ammonites have been found in six areas in southern Alaska over a distance of about 500 mi (800 km) from the Wrangell Mountains on the east to the Wide Bay area of the Alaska Peninsula on the west. In the Wrangell Mountains, some float obtained near the base of the Nizina Mountain Formation has furnished the ammonites Chondrocerax, Normatniitex, and Tehccras, which constitute good evidence for a late middle Bajocian age not older than the upper part of theS^Mawocerax hnmphriexiannm zone. In the Nelchina area of the Talkeetna Mountains northeast of Anchorage, the Tuxedni Group has furnished ammonites ranging in age from latest early Bajocian to early late Bajocian. These ammonites from the base upward include (1) Eri/citoides howelli (White), representing the Graphoce rax concarum zone; (2) Sonninia (Enhoploccrax) bifurcata Westermann, representing the lower part of the S hinctex, and Chondrocerax, representing the Stephanoceras humphriexiannni zone; and (5) Megasphrieroeerax, Sphaeroceran, Cadom itcx, and Nornta n n itex, representing the topmost Strenocerax xubfurcataw zone of early late Bajocian age. An even more complete ammonite succession of early to middle Bajocian age has been found on the west side of Cook Inlet between Tuxedni Bay and the eastern part of thelniskin Peninsula just west of Chinitna Bay. Basally that succession differs from the succession in the Talkeetna Mountains by the presence of Tmetocerax xcixxnm (Benecke) both below and along with occurrences of Eri/citoidex hoici'Ui (White) and by the presence of Emileia, Sonninia, and S. (Papilliccrax) between the underlying beds containing Parabigotitex and the overlying beds containing Teloeerax, Norman n itex, and Chondrocerax. The presence of Tmetoceras below the occurrences of Erijcitoidex favors an age slightly older than the Graphocerax conearn m zone of latest early Bajocian age. The presence of Sonninia and Em ileia is evidence of an age not younger than the zone of Otoitex xaiizei. The Bajocian ammonite succession at Wide Bay is essentially the same as that on the west side of Cook Inlet as high as the bed containing Sonninia tii.rednienxix Imlay. The succession at Wide Bay, however, has furnished more genera and species and has made possible a threefold faunule division of the beds characterized by Docidocerax (Pxeiidocidocerax) iridebayenxe. INTRODUCTION Most of the early and middle Bajocian (Middle Jurassic) ammonite taxa present in southern Alaska were described by the writer in 1964 and by Gerd Westerman in 1964 and 1969 (figs. 1-7). The writer at that time dealt mainly with ammonites collected either northwest of Cook Inlet between Tuxedni Bay and Iniskin Bay or in the eastern part of the Nelchina area of the Talkeetna Mountains. Gerd Westermann at those times dealt mainly with ammonites of late early to early middle Bajocian age that had been collected near Wide Bay on the Alaska Peninsula. Most of those collections had been made by field geologists working for the U.S. Geological Survey, as listed by Imlay (1964, p. Bl), who also made extensive collections from Bajocian sedimentary rocks in southern Alaska, in 1948, 1952, 1962, 1972, 1974, and 1977. The stratigraphic occurrences of the ammonites (figs. 8-13) are based mainly on studies by R. L. Detterman (1963) and by J. K. Hartsock (1966, p. 20-34) in the area west of Cook Inlet; by Arthur Grantz (1965) in the Nelchina area of the Talkeetna Mountains; by E. M. MacKevett, Jr. (1971, p. 16,17), in the Wrangell Mountains; and by R. L. Detterman and associates (1977, 1980, and 1981) in the Paule Bay and Wide Bay areas of the Alaska Peninsula. Only those taxa are described herein that have not previously been identified or described, or for which new biologic, stratigraphic, or geographic data are now available. BIOLOGICAL ANALYSIS The U.S. Geological Survey's fossil collections from southern Alaska contain at least 1,224 ammonite specimens of early and middle Bajocian age. Their distribution by genus, subgenus, subfamily, and family is shown in table 1. Among the families, the Hammatoceratidae comprise about 28% percent of the total number of specimens; the Stephaneroceratidae, nearly 19 percent; the Sonniniidae, 8% percent; the Sphaeroceratidae and Perisphinctidae, each a little more than 8 percent; the Oppeliidae, 7% percent; the Otoitidae, 6% percent; the Phylloceratidae and Hildoceratidae, each EARLY AND MIDDLE BAJOCIAN AMMONITES FROM SOUTHERN ALASKA