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Characteristic Jurassic Mollusks from Northern Alaska

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Characteristic Jurassic Mollusks from Northern Alaska Characteristic Jurassic Mollusks From Northern Alaska GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-D Characteristic Jurassic Mollusks From Northern Alaska By RALPH W. IMLAY A SHORTER CONTRIBUTION TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 274-D A study showing that the northern Alaskan faunal succession agrees with that elsewhere in the Boreal region and in other parts of North America and in northwest Europe UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1955 UNITED STATES DEPARTMENT OF THE INTERIOR Douglas McKay, Secretary GEOLOGICAL SURVEY W. E. Wrather, Director For sale by the Superintendent of Documents, U. S. Government Printing Office Washington 25, D. C. - BMMH§ts (paper cover) Price $1.00 CONTENTS Page Abstract_________________ 69 Introduction _________________ 69 Biologic analysis____________ 69 Stratigraphic summary. _______ 70 Ages of fossils________________ 73 Comparisons with other faunas. 75 Ecological considerations___ _ 75 Geographic distribution____. 78 Summary of results ___________ 81 Systematic descriptions__ _. 82 Literature cited____________ 92 Index_____________________ 95 ILLUSTRATIONS [Plates &-13 follow Index] PLATE 8. Inoceramus and Gryphaea 9. Aucella 10. Amaltheus, Dactylioceras, "Arietites," Phylloceras, and Posidonia 11. Ludwigella, Dactylioceras, and Harpoceras. 12. Pseudocadoceras, Arcticoceras, Amoeboceras, Tmetoceras, Coeloceras, and Pseudolioceras 13. Reineckeia, Erycites, and Cylindroteuthis. Page FIGXTKE 20. Index map showing Jurassic fossil collection localities in northern Alaska.. 79 TABLES TABLE 1. Relative abundance of specimens of Jurassic molluscan genera in northern Alaska. 70 2. Geographic distribution of Jurassic macrofossils from outcrops in northern Alaska.. 77 3. Test wells from which Jurassic fossils and stratigraphic data have been obtained- _ _ 82 4. Jurassic macrofossils from well cores in northern Alaska______________________ 82 CHARTS CHABT 1. Correlation of Jurassic formations of northern Alaska_____ In pocket A SHORTER CONTRIBUTION TO GENERAL GEOLOGY CHARACTERISTIC JURASSIC MOLLUSKS FROM NORTHERN ALASKA BY RALPH W. IMLAY ABSTRACT Leffingwell in 1911 and by field parties from the United The fossils from the Jurassic strata of northern Alaska prove States Geological Survey since 1947. During the sum­ that the Lower, Middle, and Upper Jurassic series are repre­ mer of 1950 the writer spent 10 days along the Canning sented but suggest that certain stages or parts of stages are Eiver and 1 day along the Sagavanirktok Eiver ex­ not represented. There is no faunal evidence for the presence of the middle and upper parts of the Bajocian, the entire amining Jurassic outcrops. This provided some first­ Bathonian, the upper part of the Callovian, the lower Oxfordian, hand field information concerning the difficulties of or the upper Portlandian. Field evidence shows that a discon- mapping the Jurassic strata and of determining the formity occurs at the stratigraphic position of the upper Port­ stratigraphic succession. The brief descriptions of the landian. Both field and subsurface data suggest an uncon­ stratigraphic and lithologic relationships presented formity immediately preceding the upper Oxfordian. The herein as background for the ecologic and stratigraphic absence of faunal evidence for certain stages, or parts of stages, may be related to the fact that elsewhere in Alaska and in the interpretations have been obtained mainly from dis­ western interior of North America major retreats of Jurassic cussions with the geologists who have recently studied seas occured during Bathonian, late Callovian, and Portlandian the Jurassic rocks. These geologists, including George times. Gryc, C. L. Whittington, I. L. Tailleur, E. G. Sable, Although the Jurassic strata in northern Alaska are gen­ A. S. Keller, W. W. Patton, Jr., B. H. Kent, M. D. erally impoverished faunally, nevertheless, in many places in­ terpretations of the stratigraphy or the structure are based on Mangus, J. T. Dutro, H. N. Eeiser, E. L. Detterman, the fossils present, or the fossils are used as supplementary and E. H. Morris, have also checked or rewritten all evidence. Wherever the faunal succession can be determined locality descriptions and have plotted the positions of in northern Alaska, it agrees essentially with that elsewhere in the localities on figure 20. Their efforts have contrib­ the Boreal region and in other parts of North America and uted greatly toward increasing the accuracy and value in northwest Europe. Faunal and lithologic relationships suggest that the eastward- of the data in this report. It is hoped that the inter­ trending Jurassic seaway of northern Alaska had rather uni­ pretations of the data will have immediate use in north­ form and moderately steep slopes along its northern and ern Alaska and will have application in other parts of southern margins and that more than half of its sea bottom Alaska. was stagnant and at least as deep as the lower part of the BIOLOGIC ANALYSIS neritic zone. The existence of moderately deep water may ex­ plain the presence of the ammonites Phylloceras, Lytoceras, The Jurassic fossils from northern Alaska that are and Reineckeia, which are missing in the shallow-water Jurassic strata in the interior of North America, in east Greenland, and generically identifiable include 163 specimens of am­ in the Barents Sea area. The scantiness of the fauna over much monites, 9 belemnites, 530 pelecypods, 5 scaphopods, 9 of the seaway is problably related to unfavorable bottom condi­ rhynchonellid brachiopods, and more than a hundred tions and to an inadequate supply of certain materials such pieces of the crinoid Pentafirinus. Of the total of 707 as phosphate. Fairly warm waters during Early Jurassic and individual mollusks at hand, 338 belong to the pelecy- early Middle Jurassic (Bajocian) time is indicated by the presence of ammonites that had a nearly worldwide distribution. pod Aucella, and 369 belong to other genera. The Somewhat cooler waters and the presence of climatic zones Aucellas are a mere sampling of the enormous numbers during the Late Jurassic in Alaska, as in other parts of the present in the outcrops, but the other 369 mollusks Boreal region, is indicated by the presence of molluscan genera represent nearly all that could be obtained by the field quite distinct from those in the Late Jurassic of the Mediter­ geologists in the time available. This number is sur­ ranean region. prisingly small considering that many field parties have INTRODUCTION worked in the area during the past 50 years. It shows This study of the Jurassic macrofossils from north­ that the outcropping Jurassic strata are poorly fos- ern Alaska is based on collections made by E. de K. siliferous except for Aucella. The number of specimens 69 70 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY in the molluscan genera arranged according to their such as Gra/mmatodon, G-ryphaea, and Pholadorwfa are age is shown on table 1. known by single occurrences. Actually the only fairly Many of the fossils in the collections are either common pelecypods are Inoceramus and Aucella, and crushed or fragmentary. Most of the pelecypods, other these characteristically do not occur together. than Aucella, are so imperfectly preserved that they Among ammonites, too, very few genera are repre­ are not worthy of description and are merely listed sented, considering the amount of Jurassic time and generically. Most of the ammonites are merely com­ strata present. It seems probable, however, that the pared specifically with European or American species. subsurface Jurassic contains a greater variety of am­ Three ammonites are identified specifically with am­ monites than the outcrops, judging by the few cores monites occurring in southwestern Alaska. Four that have been obtained. Most of the ammonite genera species of Aucella, are identified with species that are present in Alaskan Jurassic rocks are fairly common common in Boreal Jurassic strata. Not a single species elsewhere in Boreal Jurassic strata and in northwest is described as new. Partly, this reflects the wretched Europe, but the presence of Phylloceras, Lytoceras^ and preservation of the fossils. It is probable, however, Reineckeia north of the Arctic Circle in Jurassic de­ that better preservation would allow positive identifi­ posits is unique (Spath, 1932, p. 149, 151). This is cations of a number of species with named species probably the first record of Lytoceras and Reineckeia elsewhere in North America, in the Boreal region, or from Jurassic rocks bordering the Arctic Ocean. The in northwest Europe. record of a Reineckeia is particularly interesting, be­ cause this genus has been considered as a characteristic TABLE 1.—Relative abundance of specimens of Jurassic molluscan element of Mediterranean faunas. genera in northern Alaska STRATIGRAPHIC SUMMARY Lower Lower Oxfordian and Genera Jurassic Bajocian Callovian Kimmeridgian rocks rocks rocks rocks Detailed descriptions of the distribution, thicknesses, lithologic features, and subdivisions of the Jurassic 2 strata exposed in northern Alaska are being prepared 40 by geologists of the Alaskan Branch for publication by 1 1 the Geological Survey. Completion of such detailed 4 descriptions is essential before accurate generalized de­ Lima. ___ _________ 6 scriptions of the formations can be made or before Gryphaea __ _______ 9 14 10 well-substantiated interpretations of the origin and re­ 338 gional stratigraphic relationships of the formations can 40 be formulated. For the
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