map of northern Alaska showing duadrangles referred to in text, locations of dgures 27, 29, and 29, generalized outcrop belt of Mississippian rocks, and approximate

distribution of Permian ro<k naits.

LATE PALEOZOIC GASTROPOD

FROM NORTHERN ALASKA

STRATIGRAPHIC DISTRIBUTION OF THE GASTRO~ODS

The asseniblage of gastropod genera dates the rocks as late Paleozoic, but the gastropods themselves are of little value in correlating individual stratigraphic units with rock sequences in regions outside Alaska. AVith the exception of Po> tloclit'ella sp., Rl<zneo- n'erma'> sp., and Tap bonellina? lata, n. Sp.i Ilo g Ilei'a or species thought, to be. limitecl to rocks of Mississip- piiln age are known. No species or genera diagnostic of. Perniian age were identified.

On the other hancl, fieldwork has denionstrated that some of the. gastropods are, useful, locally, in provid- ing supplementary evidence that helps determine the position of ceitain rock units. ITnits currently rec- ognized in northern Alaska are discussed below on the basis of distribution of the gastropods.

ROCK UNITS

SHORTER CONTRIBUTIONS

TO GENERAL GEOLOGY

C. C. Branson's bibliographic index of Permian fos- sils described through 1041 (Branson„1M8) aids a, search of the literature for previously namecl species. Mississippian fossils, on the other hand, have not been. systematically indexed for many years. To the best of the writers' knowledge, the last niajor papers deal- ing with gastropods of Mississippian age or early Carboniferous, are those of I<"oninck (1881, 1883). Literature clescribing Anierican Mississippian gastro- pods ls scaIlt.

The Brooks Range physiographic province includes several clistinct groups of mountains extencling from near the 1%th nieiiclian eastward to the, Canadian boundary. For most of its length the range lies es- sentially along the (>8th parallel, foriiiing the back- bone of Alaska, angl constituting the divide for clrain- age to the Pacific and Arctic Oceans. Eastward froni the lr>0th meridian the range niakes a crescentic bend northeastivarcl until the, mountain front is only about, 25 niiles south of the Arctic coast. The, regional set- ting and topography were described in detail by Sniith angl Mertie. (10>O) ancl sunilnarizecl by Payne and others (1051). The inclex lnap (fig. >8) shows the quadrangles froui which gastropod speciniens were collected,

The topography of the Brooks Range has been ex- tensively Inodified by glacial actiou, Gl<iciers are not uow couunon, but the cliniate reniains severe. The isolated position of the region has complicated aud handicapped fossil collect,ing. Many collectious were backpacked by geologists froln the collecting locali- ties and cachef1 in places accessible by airplane. This necessarily liinited the nun>her and. size of the fossil collections. A feiv of the localities ivere visitecl more than once; niost have been only casually san>pled.

More than custoniary ackliowledglnent, is clue the, fieklnien because fieklwork in northern Alaska is rig- orous 'lncl requires phVslclil e(fort llot 1101'nlally asso- ciatecl with fossil collecting. Menibers, or fornier nielubers, of the IT.S. (reological Survey who collected fossil material on ivhich this paper is basecl are: A. L, Bowsher, 3V. P. Brosge, H. M. Chapman, H. L. Det- ternian, J. T. Dutro,,Ir., Allen Feder, W. A. Fischer, George Criyc, C..7. Crudiru, A. S. Iveller, B. H. I< ent, C. E. I< irschner, A. H, Lachenbruch, M. D. Mangus, Pt. H. Morris, >iV. AV. Patton, Jr., H. N. Reiser, E. G. Sable, I. L. Tailleur, angl H. F. Thurrell, Jr.

Dr..J. Brookes I< night, Sniithsonian Institution, re- tired, exauiinecl some of the speciniens aud niade per- tinent taxononiic suggestions. Photographs weiv taken by Nelson ItV. Shupe, IT.S. Geological Survey.

Hocks of hite, Paleozoic age in northern Alasl-a can be assigned to at least six formations. The Missis- sippian systeni, represented largely by a con>plex ar- rav of carbonate facies, is divided into two major parts. The nonienclature of Mississippian rock units in the central Brooks Range (fig. A>) has been re- vised by Bowsher and Dutro (1057, p. 8 — 7). The lower part, essentially a, black shale with a, sanclstone meniber at the base and argillaceous limestone beds near the top, is flesignatecl. as the Ivayak shale. The upper part, the Lisburne group, consists of the Wachs- uiuth limestone, below and the Alapah limestone above. Elsewhere in noithern Alaska, the Lisburne group has been subdivided into several forniations, not as yet formally published. The approxiniate limits of the Mississippian outcrop belt are shown in figure 28.

No rocks of undoubted Pennsylvauian age are known froni northern Alaska. A carbonate rock sequence in the eastern Brooks Range, lying above the Alapah limestone, may represent some pait of the Pennsyl- vilnian systeni. Fossils are rare in these rocks and no gastropods have been collected from theni.

The Permian is represented by three, quite diferent fornuitions„each of which represeuts a pi>dominantly clastic facies, The approxiniate limits of outcrop of these units are shown in figure 08. The Sadlerochit formation of Permian and Early Triassic age in the e;lstern Brooks Range was first, defined as the Sad.- lerochit sandstone by Leffingwell (1010, p. 103). Its fauna, was stuclied by Gii<y and, although the fc"ma- tion was originally called Pennsylvanian, it, was Girty who recognized amenities with f'aunas now considered to be of Permian age (Smith, 1030, p. M). The Sik- sikpuk formation of Permian{2) age in the central Brooks Range was described recently by W. W. Pat-