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AAPG Bulletin

Abstract


Volume: 51 (1967)

Issue: 6. (June)

First Page: 849

Last Page: 863

Title: Paleoclimatic Interpretations of Some Mesozoic Floral Sequences

Author(s): Charles J. Smiley (2)

Abstract:

Studies of Cretaceous floral sequences in northern Alaska were conducted during five field seasons from 1956 to 1966. Seven study areas across more than 350 miles of the Arctic Slope were examined in considerable stratigraphic detail. Marine invertebrate index fossils collected from deposits interbedded or intertonguing with plant-bearing units permit references to European stages of the Cretaceous. Comparisons of local floral sequences permit regional correlations of non-marine deposits across northern Alaska. Vegetational changes noted through thousands of feet of section in the various study areas provide a basis for inferring regional climates and climatic trends. Marine faunal interties indicate a time span from middle Albian to probably Maestrichtian, about 45 to 50 million years.

Comparisons with selected North American Mesozoic floras and floral sequences at high latitudes (about 70° N.) and at middle latitudes (about 40° N.), and with Jurassic and Cretaceous records of Eurasia, suggest that vegetational and inferred climatic zonations were latitudinal as at present. Evidence suggests that vegetational zones shifted northward from Early Jurassic to medial Cretaceous (Albian) time, indicating a warming trend, followed by cooler climates in later Mesozoic time. Comparisons with floral records from other areas of the Northern Hemisphere, and of inferred climatic changes, do not favor concepts of wandering poles, drifting continents, or change in axial inclination as causal factors of climatic change since at least the beginning of Jurassic time.

Text:

INTRODUCTION

Several areas across more than 350 miles of northern Alaska contain river-cutbank and coastal-cliff exposures with excellent stratigraphic sequences of Cretaceous fossiliferous deposits (Fig. 1). The strata have been tilted or folded, commonly occurring as homoclinal sequences on the flanks of broad folds or in fault blocks. In some areas marine and non-marine sediments intertongue, and relations between plant-bearing beds and ammonite-pelecypod beds are clearly demonstrated by superposition.

Regional geology and marine faunal correlations have been conducted by U. S. Geological Survey teams since the mid-1940s across much of northern Alaska. Several reports have been published on test-well analyses, on regional geology, and on invertebrate paleontology (see references); these have served as basic references for stratigraphic considerations of fossil beds. The geographic locations and stratigraphic positions of fossil localities were obtained through comparisons of vertical aerial photographs with geologic maps, and through comparisons of lithologic criteria and of stratal sequences. Generally it was possible during the field investigations to refer floras and faunas to specific lithologic units of the detailed measured sections described in these U. S. Geological Survey r ports.

FLORAL-FAUNAL INTERTIES, NORTHERN ALASKA

Kuk River (area 3, Fig. 1):
The project on Cretaceous paleobotany began in 1956 in connection with field research on insectiferous amber under the leadership of R. L. Langenheim, Jr. The primary goal at that time was to locate and collect amber samples in situ and to collect fossil plants for dating purposes (Langenheim et al., 1960). This initial study was in the upper (southern) part of the Kaolak-Kuk Rivers system. The project was continued in 1961, and the floral study was extended northward to Wainwright on the Arctic coast (Smiley, 1966). The study area crosses the axis of a synclinorial structure and extends outward across its uncomplicated north limb. About 4,250 feet of non-marine Cretaceous deposits is present in the local surface section. Forty representative collections of plant megafossils were found a different levels from near the

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base to the top of the exposed section. The entire floral sequence is post-middle Albian, based on marine invertebrates in Kaolak Test Well No. 1 (Bergquist in Collins, 1958; Imlay, 1961), and appears to range into Senonian. The test-well faunal zone appears to be equivalent to beds that crop out near the coast, and thus is somewhat lower in the section than the lowermost plant-bearing beds. Dating of florules from overlying deposits originally was a problem because no similar floral sequences were known then from other areas of northern Alaska.

Chandler-Colville (areas 1 and 2, Fig. 1):
The study was extended in 1964, and during part of 1966, 200 miles eastward into the Chandler and Colville Rivers region where floral-faunal interties had been reported by U.S. Geological Survey teams (Detterman et al., 1963). Rock units and fold axes can be traced from area 1 to area 2, permitting correlations by means of lateral continuity. The two study areas contain essentially equivalent sections 8,000-9,000 feet thick. Both contain interbedded marine and non-marine deposits, and the faunal records are tied physically to floral sequences (Table I). The faunal records contain a variety of ammonites and the pelecypod Inoceramus, permitting the reference of associated florules to European Cretaceous stages. Fossil collections were taken from a total of 82 levels in the two areas, and m rine faunal interties were obtained at 19 of these. Faunal evidence indicates that all stages from middle Albian probably to Maestrichtian are represented in the Chandler-Colville area, a time interval of 45-50 million years according to the time scale of Kulp (1961).

Kukpowruk River (area 4, Fig. 1):
In 1965 field studies were continued into the western part of the Arctic Slope. Study areas extend along a part of the Kukpowruk River, and along the coast near Cape Lisburne. In the Kukpowruk area two partly duplicated sections were sampled; the fossil-bearing beds were referred to specific units in the measured sections of Chapman and Sable (1960). Both sections contain interbedded marine and non-marine deposits in the lower part (Kukpowruk Formation), grading upward into continental deposits of the Corwin Formation. Fossil collections were taken from 59 levels, extending upward from the base of the Kukpowruk Formation through about 10,000 feet of section. Faunal records from 13 marine interbeds in the lower part of the section indicate a middle Albian age for the Kukpowruk Formation. loral comparisons with the Kuk River and the Chandler-Colville sequences indicate a late Albian age for

Fig. 1. Index map of Alaskan study areas north of Brooks Range. Approximate latitudes 69° to 71° N.

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units in the lower part of the overlying Corwin Formation, and a probable early Cenomanian age for units in the uppermost part of the section (at the 10,000-foot level).

Punak Creek (area 5, Fig. 1):
Coastal study areas include the mouth of Punak Creek where a single floral collection was taken from units mapped by Chapman and Sable (1960) as the Kukpowruk Formation. By relating the structural attitudes of the strata to the position of the Kukpowruk-Corwin contact on Chapman and Sable's map, it was determined that the flora is at a level more than 2,000 feet below the contact. The Punak Creek flora is similar to ones collected from lower beds of the Corwin Formation in the Kukpowruk River area where the Corwin is in gradational contact with the type Kukpowruk, rather than to ones from subjacent Kukpowruk beds.

Pitmegea River (area 6, Fig. 1):
From the mouth of the Pitmegea River for about 1 mile upstream are exposures of tilted beds in homoclinal sequence about 1,500 feet thick. This section appears to be equivalent to the lower portion (part of the Silty Shale and Lower Sandstone members) of the type Corwin section 10 miles west. A sequence of 16 florules was collected; no marine invertebrates were found. Floral comparisons with other study areas indicate an Albian age for material from all levels in the Pitmegea sequence.

Corwin Bluff (area 7, Fig. 1):
Work was begun in 1965 on the 15,500-foot type Corwin section, and the study was completed during the summer of 1966. This section is exposed through a distance of about 10 miles in coastal bluffs east of Cape Lisburne. A sequence of 57 florules was taken through the Silty Shale, Lower Sandstone, Shale, Coal and Sandstone, and Conglomerate members of the formation. Parts of the Bentonitic Clay member were snow-covered in 1966, but a sequence of four florules was found in the exposed middle part of the unit. The Upper Sandstone member was largely inaccessible in 1966 because of snow cover, but specimens from large blocks at the base of the bluffs below the snow banks, and a florule from exposed beds in the middle of the unit, provided some evidence of the floral composition in this upperm st member. Fossils collected during 1965 and 1966 thus represent an essentially continuous floral record through the entire 15,500 feet of the type Corwin section. Floral comparisons suggest an Albian age for all members of the Corwin Formation in the type area.

GENERAL PALEOBOTANY, NORTHERN ALASKA

Introductory statement:
Taxonomic analyses have been conducted largely on a generic or higher level. More detailed studies of museum material representing later Mesozoic floras from other parts of the world have been deferred until completion of the field data-collecting phase of the project in 1966. It is believed that conclusion of the Corwin study provides data of sufficient geographic, stratigraphic, and taxonomic scope to serve as a sound factual basis for considerations of Cretaceous vegetation.

Plant megafossils include imprints, molds and casts, and carbonized remains of megascopic organs. Wood chips, stems, and small logs are common in some beds, but details for identification are lacking. Tree stumps perpendicular to bedding planes (and thus assumed to be in original position of growth) may be found in a few places; these indicate the size of woody plants that grew at the site of deposition: 18-24 inches known maximum diameter. Identifiable megafossils include fern fronds (some of which have both fertile and sterile pinnae on the same specimen), conifer shoots and cones that commonly are attached to leafy shoots, and compound angiosperm leaves with leaflets still attached to the petiole. As plant organs do not remain in such condition during long periods of transport, it s assumed that the remains represent the vegetation which grew in the vicinity.

Judging from the intertonguing relations of marine and continental sediments in many of the areas sampled, and from the common and widespread occurrence of coal deposits in non-marine sequences, it appears that, except for minor fluctuations, the region generally was a subsiding marginal shelf during most of Cretaceous time. Rates of subsidence and of sedimentary accumulation appear to have coincided to the degree that the surface of accumulated sediments remained at or near sea-level during this interval of time. A humid, commonly swampy, coastal-plain environment is inferred for vegetation at all stratigraphic levels.

General taxonomic considerations:
Ferns are

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common and varied in Albian and early Cenomanian deposits, but are uncommon and of few kinds in later floras.

Cycadophytes are common and varied in rocks of Albian age, but they are rare and of one or two types in Cenomanian floras. Members of this group have not been found in post-Cenomanian florules in northern Alaska.

Ginkgophytes in Albian and Cenomanian deposits are represented by deeply dissected or linear leaves of several genera, and by digitate leaves of Ginkgo (Ginkgoites vide Seward). Post-Cenomanian floras contain only entire or bilobate leaves that are characteristic of the living G. biloba Linne.

Conifers in pre-Cenomanian beds are represented by primitive-appearing forms similar to but in few cases referable to modern genera. Most conifers that are referable to living genera first appear in Cenomanian or younger floras.

Angiosperms occur at Albian levels only as scarce leaves of one or two species. They appear to be referable to such families as Lauraceae and Sterculiaceae, both of which range into warm-temperate climates. Floras of late Albian or early Cenomanian age mark the appearance of varied angiosperms as important though not dominant elements of florules, members of the Platanaceae being characteristic. Cenomanian vegetation changed from (a) an early association of angiosperms, ferns, and assorted gymnosperms that are more characteristic of older floras to (b) a later association of angiosperms and conifers of mixed primitive and modern types. In younger deposits new angiosperms assume a progressively more dominant role as older, more primitive taxa are replaced.

Biostratigraphic considerations:
Distinct changes in vegetation are noted from the bottom to the top of all sections studied in northern Alaska (Table I).

Albian floras are characterized by an abundance

Table I. FLORAL-FAUNAL INTERTIES IN ARCTIC ALASKA

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and variety of ferns, cycadophytes, dissected-leafed ginkgophytes, and primitive conifers. Angiosperms usually are absent in florules of Albian age, and are represented only by a few leaves, except in late Albian florules where one or two species may be represented by several leaves.

Cenomanian vegetation is characterized by a mixture of primitive and more modern conifers, by a large number and variety of angiosperms, and by ginkgophytes with dissected leaves. Ferns are of lesser importance than in Albian floras, and cycadophytes are reduced to the status of a few minor elements.

Turonian and younger floras are dominated by angiosperms and modern-appearing conifers. Modern-type Ginkgo (Ginkgoites) and ferns are minor elements, and cycadophytes and dissected-leafed ginkgoids are absent. Turonian deposits appear to be characterized by an association of varied angiosperms, the last record of the spatulate-leafed genus Podozamites, and modern conifers representing the Taxaceae and the Taxodiaceae. Senonian floras also are angiosperm-dominated, including the form Trochodendroides, in association with mainly Taxodiaceous conifers and non-digitate leaves of Ginkgo (Ginkgoites). Maestrichtian(?) floras, poorly represented in the areas studied, are dominated by angiosperms associated with Taxodiaceous conifers and nondigitate leaves of ginkgophytes. Ferns are present b t are rare as megafossils.

Concluding statement:
Close attention has been given to the stratigraphic position of fossil-bearing beds in local sections. Superpositional relations are the chief factor for determining their relative ages, and local sequences of florules are thus clearly documented in the field by physical criteria. Marine interties providing sequences of excellent index fossils afford criteria that are independent of floral evidence for reference to European stages.

Table I shows that plant-bearing continental deposits can be referred to stages of the Cretaceous, providing representative collections of plant megafossils are available. All plant categories are limited stratigraphically in some degree, and some species may be restricted in northern Alaska to a single stage or perhaps part of a stage. Considering the numerous taxa involved and their differing stratigraphic ranges, it appears that plant megafossil records may be very effective for precise correlation of non-marine deposits across the Arctic Slope of Alaska.

COMPARISONS WITH OTHER REGIONS

Introductory statement:
Although Mesozoic floras of North America have not been integrated recently in the comprehensive manner of Vakhrameev (1964) for the Eurasian continent, records from selected areas of North America do provide significant data for making inferences regarding vegetational and climatic conditions (Fig. 2). Of importance at high latitudes (about 70° N.) are the floral sequences across northern Alaska (e.g., Smiley, 1966), and the Cretaceous sequence on the west coast of Greenland (Koch, 1964). At middle latitudes (about 40° N.) are the sequences of the Potomac and younger Cretaceous floras on the east coast of the United States (Dorf, 1952), a medial Cretaceous sequence in the Mid-Continent (Fontaine, 1893; Berry, 1922), and the "Jurassic" flora of Oregon on the west coast (Fontain , in Ward, 1905).

Northern Alaska summary:
The floral sequences in the several areas across the Arctic Slope of Alaska have been discussed on preceding pages. These areas are north of the Brooks Range, and extend from the Chandler River westward more than 350 miles to Cape Lisburne. The Alaskan data suggest that, for any particular part of the Cretaceous, climatic conditions were uniform across the coastal plain. They also suggest that the regional climate was changing during this interval of time. Comparisons between Cretaceous and living plant taxa lead to the conclusion that (see Smiley, 1966): (1) living relatives of Albian plants in northern Alaska are now confined to latitudes south of about 30° N., generally at moderate elevations = humid warmer climatic conditions; (2) modern relatives of later Cretaceous plants are onfined mainly to latitudes of about 35°-45° N. = humid temperate climatic conditions; and (3) a cooling trend occurred in this region from Albian time through the later stages of the Cretaceous Period.

West Greenland:
Koch (1964) recently has summarized the stratigraphic problems and the probable floral sequence in Cretaceous deposits of the Disko Island-Nugssuaq Peninsula area of west Greenland, 2,000-2,500 miles east of the Alaskan area (see also Imlay and Reeside, 1954). Though physical-stratigraphic control seems to be largely

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lacking in west Greenland, the four floral types recognized by Koch appear to have correlatives in the sequence of Cretaceous floras in northern Alaska (Table II).

The gymnosperm- and fern-dominated Kome flora, considered to be the oldest in the Greenland area ("lower Cretaceous"), represents vegetation similar to that found in Albian beds of the Alaskan region. The angiosperm-dominated flora of the Pautut (Patoot), having marine interbeds containing a Senonian fauna, represents vegetation similar to Senonian floras of Alaska. Koch's Upernivik Naes flora, which he considers to be intermediate between the Kome and Pautut floras, is gymnosperm- and fern-dominated with varieties of cycadophytes and a single angiosperm species of Platanaceous affinities that may be important numerically. This would appear to represent a stage of floral development about equivalent to that found in the middle part of the Killik Tongue (Chandler Formation) in the Chan ler River area, and in upper Kuk florules in the Kuk River area (floral unit C of Smiley, 1966). The Atane flora of west Greenland is thought by Koch, on floristic grounds, to be somewhat younger than the Pautut flora, both of which are angiosperm-dominated.

The west Greenland record proposed by Koch

Fig. 2. Map of North America showing selected areas of Cretaceous floras and floral sequences, at high latitudes (about 70° N.) and at middle latitudes (about 40° N.). 1-Northern Alaska. 2-West Greenland. 3-Atlantic Coast of United States. 4-Mid-Continent United States (Kansas area). 5-Southwest Oregon (?Jurassic).

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appears to represent a medial to later Cretaceous floral sequence that in general parallels the vegetational development in northern Alaska. A marine connection between the floral sequences of the two regions can be established by faunas containing the Senonian species Inoceramus patootensis. This pelecypod occurs in Pautut beds of Greenland and in the marine Schrader Bluff Formation that intertongues with the plant-bearing Prince Creek Formation in northern Alaska. The species also has been reported in marine deposits of the Soviet Arctic, where it has been correlated with units containing floras similar to those of Senonian age in Alaska and Greenland (Sachs and Strelkov, 1961).

East Coast United States:
Dorf (1952) discussed the stratigraphy and floral sequences in the Potomac Group and younger units of the east coast of the United States. His Table 1, Figure 1, and intervening species lists (p. 2164-2169) show three general floral types. (1) The first is the Patuxent (early Neocomian) and Arundel (Aptian), composed predominantly of gymnosperms, and ferns, with numerous and varied cycadophytes. Ginkgophytes are scarce and have deeply dissected leaves (Baiera). Angiosperms, represented by two species in a single genus (Ficophyllum), appear indicative of warm climatic conditions. (2) The second is the Patapsco (Albian) composed of a mixture of ferns, gymnosperms, and angiosperms. Floras of this age appear to contain fewer cycadophytes and more conifers than earlier ones of this region. An iosperms are represented by a variety of forms that appear to be indicative of warmer climatic conditions (Menispermites, Sapindopsis, Celastrophyllum, Cissites, Sassafras, Sterculia); a few are apparently of more temperate character (Populus and Populophyllum, Quercophyllum, Ulmophyllum). (3) The last is the Raritan (medial Cenomanian to early Turonian) and Magothy (later Turonian and Coniacian), which are angiosperm-dominated floras. Conifers appear to represent more temperate climatic conditions than those in earlier floras, and cycadophytes are of minor importance. The angiosperms are represented by a mixture of warmer and more temperate taxa.

This mid-latitude sequence of Cretaceous floras outlined by Dorf suggests that here, as in Arctic regions, Albian and older vegetation of warmer character is replaced in later Cretaceous time by floras that are indicative of more temperate climates.

Mid-Continent United States (Kansas area):
This area contains the earliest known mid-latitude record of an angiosperm flora associated with a fossiliferous marine unit. The plant-bearing Cheyenne Sandstone is conformably overlain by the marine Kiowa Shale of middle Albian age (Cobban and Reeside, 1952). The somewhat older Trinity Group, dated early to middle Albian, contains a flora dominated by gymnosperms.

This earliest mid-latitude record of an angiosperm-dominated flora, dated no younger than middle Albian by overlying fossiliferous marine deposits, contrasts with records of contemporaneous floras 30° farther north. In Arctic Alaska floras dominated by angiosperms are not recorded until Cenomanian-Turonian times, millions of years later. Though remains of angiosperms are present in later Albian floras of the Arctic, they are of minor importance or, if numerically prominent, are represented by one or a few species in a flora that is otherwise dominated by gymnosperms and ferns.

The Cheyenne flora seems to represent a stage of floral development about equivalent to the Patapsco flora in the Atlantic coastal sequence on the east. Such evidence supports the view (see also Axelrod, 1959) that angiosperms began to dominate vegetation of lowland depositional sites at middle latitudes millions of years before they invaded the lowlands of the Arctic.

West Coast United States:
Fontaine (in Ward, 1905) described and illustrated a gymnosperm-flora of presumed Jurassic age from the Buck Mountain area of southwestern Oregon. This flora is from about the same latitude as the Mid-Continent and Atlantic coastal areas farther east in the United States. The Oregon flora is dominated by a variety of ferns and cycadophytes, and by digitate leaves of Ginkgo (Ginkgoites). The numerically high representation of these ginkgophytes contrasts strikingly with the Mesozoic floras in other mid-latitude areas, and suggests a much closer relation with Mesozoic floras of Alaska and Siberia. Several of the fern, cycadophyte, and ginkgophyte forms also are found in Alaska where their youngest occurrence is in floras of Albian age.

Soviet Arctic:
Floral sequences in northern Eurasia, similar to those of Arctic Alaska, have

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been studied during the last few years by Soviet paleobotanists. These workers recently have published monographic reports on floras from areas across the Soviet Arctic, and from different stages of the Jurassic and Cretaceous. Illustrations of fossil specimens indicate a high proportion of probably identical species in Cretaceous floras of northeastern Eurasia and Alaska. The floristic resemblance is so close that there is little doubt that the Cretaceous vegetation in northwestern America is an extension of that which occupied the Soviet Arctic.

Samylina (1963) described a floral sequence collected by Vakhrameev along the Aldan River, a tributary of the Lena River 1,500-2,000 miles west of the northern Alaska area. The illustrated specimens show an affinity with older floral types from northern Alaska. The following excerpts from the English summary (p. 139) indicate a close attention to detailed field stratigraphy and to local floral sequences, similar to that of the Alaskan studies.

The Mesozoic flora of the Aldan and its lower tributaries in its lower course is one of the richest among the hitherto known Mesozoic floras of Siberia. ^hellip The collections of the remains of plants from successive strata enabled the author to trace the changes in the composition of the vegetation through the entire geologic section, three complexes of fossil plants having been distinguished^hellip The Aldan Mesozoic flora is a typical representative of the floras of the Siberian paleofloristic region. It existed under conditions of moderately moist warm-temperate climate with succession of season^hellip During the Upper Jurassic and Lower Cretaceous the Aldan flora was developing in the direction of a considerable increase of the proportion of Cycadales and Bennettitales and the d crease of the proportion of Ginkgoales and to a smaller extent the Filicales and Equisetales^hellip It is possible that all these changes are due to the warming of the climate during the Lower Cretaceous. (Italics by C.J.S.)

Floral sequences of later Cretaceous time (Vakhrameev, 1958; Sachs and Strelkov, 1961) appear to parallel those of equivalent ages in northern Alaska and in Greenland. The evidence suggests that vegetational changes were Holarctic in scope, and that climates were cooling throughout the region during the later part of the Cretaceous.

Vakhrameev (1964) summarized the evidence of many Mesozoic floras that are widely distributed over the Eurasian continent, ranging in age from Early Jurassic to medial Cretaceous (Aptian-Albian). He figured a series of five maps for Lower Jurassic (p. 13), Middle Jurassic (p. 70), Upper Jurassic (p. 113), Neocomian (p. 134), and Aptian-Albian (p. 135). On each map are plotted the locations of floras of that age, with symbols indicating whether they represented elements of a northern (cooler) or a southern (warmer) floral type. From these comprehensive studies, Vakhrameev distinguished two broad vegetation zones for the Eurasian continent: a northern "Siberian Paleofloristic Province" and a southern "Indo-European Paleofloristic Province." On each of his maps he has drawn a boundary li e between the two provinces, approximating a zone of floral transition. The boundary lines for the five ages considered by Vakhrameev have been replotted on a single map in Figure 3.

Figure 3 shows the essentially parallel alignments of the several boundary lines, and their northward displacement during the Early Jurassic to medial Cretaceous interval. Also noteworthy are their apparent northward deflections near the margins of the continent. There is a remarkable resemblance between the lines plotted by Vakhrameev for the Jurassic and Cretaceous intervals and the configurations of lines which Chaney (1940) has shown for Eocene isoflors (p. 498) and for modern isotherms (p. 490) of the Northern Hemisphere. Chaney's isoflor and isotherm lines are replotted on a single map in Figure 4.

Assuming that Vakhrameev's lines indicate a general latitudinal zonation between two broad vegetation types (similar to forest zonation of the present), and that there was a northward deflection near the margins of the continent (also as occurs today), several conclusions may be made. (1) Prevailing westerly winds off oceans, or warm ocean currents, influenced conditions on adjacent land areas during the Jurassic and Cretaceous in much the same way that they do today. (2) There was an Atlantic Ocean or its equivalent in middle and later Mesozoic times as there is today. (3) The earth was rotating not much differently from today. (4) Finally, the Eurasian continent was in essentially the same position during Jurassic and Cretaceous times as it was during the Eocene and is today.

The northward shift of latitudinally controlled vegetation zones indicates a warming trend from Early Jurassic to medial Cretaceous, which conforms with Samylina's interpretations of the Lena River (Aldan) floral sequence. Sequential floras of later Cretaceous age in the Soviet Arctic (Vakhrameev, 1958; Sachs and Strelkov, 1961)

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suggest a later Cretaceous cooling trend, as is indicated also by the later Mesozoic floral sequences of the Alaskan Arctic and of west Greenland. Numerous workers have established that climates of Eurasia and North America began to warm again to a peak near the end of the Eocene or the beginning of the Oligocene, followed by a general cooling trend until the end of the Cenozoic.

The similarity of floral sequences and of vegetational and climatic trends on both the Eurasian and North American continents strengthens the belief that the factors causing change are of world-wide influence. From Vakhrameev's data (Fig. 3) it can be seen that all lines represent irregular concentric arcs circumscribing a central point in or near Alaska. This corresponds roughly with the general area postulated by Cox and Doell (1960) and by C. E. Helsley (oral commun., 1966) for the Cretaceous north magnetic pole based on paleomagnetic evidence. Such a coincident position, if regarded alone, might be considered a convincing argument in favor of the concept of a wandering pole of rotation. However, the same paleofloristic data also show that, during the 70-million-year interval from arly Jurassic to medial Cretaceous, there would have been little appreciable movement of the pole during a time when significant climatic changes were taking place. On the other hand, if one were to allow for a "normal" northward deflection of the lines, particularly on the western side of the continent, a center little different from the pole position of today could then be postulated. It seems most reasonable to the writer to interpret these Eurasian data to indicate a stable position of the Eurasian continent, and a stable position of the axial pole, from the beginning of the Jurassic to the present.

Latitudinal-stratigraphic considerations of angiosperms:
The Mid-Continent and east coast sequences of the United States indicate that earliest (Albian) angiosperm floras at middle latitudes (about 40° N.) were dominated by taxa of

Fig. 3. Map of Eurasia showing zonations of Mesozoic vegetation, from Vakhrameev (1964). South of boundary is Indo-European paleofloristic province. North of boundary is Siberian paleofloristic province. 1-Early Jurassic. 2-medial Jurassic. 3-later Jurassic. 4-Early Cretaceous (Neocomian). 5-medial Cretaceous (Aptian-Albian).

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warm-climatic requirements. Through subsequent stages of the Cretaceous, floras become progressively more temperate in character as taxa requiring warmer conditions disappear and as increasing numbers of more temperate-climate plants make their appearance. In Arctic regions (about 70° N.) the few angiosperms in later Albian floras are referred to taxa of warm climates. However, by the time angiosperms become important elements of the northern vegetation, they are predominantly taxa that are indicative of temperate climates (Tables II and III).

The floral sequences indicate a vegetational change, in both middle and higher latitudes, from gymnosperm-fern to angiosperm dominance. The records also show that vegetation of lowland depositional sites at middle latitudes became dominated by angiosperms of apparently warm-climate requirements during the Albian, when contemporaneous floras of Arctic lowlands were still dominated by gymnosperms and ferns. A few angiosperms of warm-climate character do occur in Albian deposits of northern regions, but their scarcity as fossils suggests that they were rare elements in the Cretaceous vegetation of higher latitudes. As climates continued to cool during the later stages of the Cretaceous, plants having more temperate-climate requirements replaced those of warmer requirements at middle lati ude lowland sites; at the same time temperate-climate forests of angiosperms began to appear in Arctic lowlands.

The above comparisons between floral records of middle and higher latitudes would seem to support several conclusions (see also Axelrod, 1966). (1) Angiosperms originated at middle or lower latitudes rather than at higher latitudes. (2) The deciduous habit probably evolved in upland areas at middle or lower latitudes, rather than in lowland sites of high latitude regions. (3) The first angiosperms to invade mid-latitude lowlands may have been those that previously inhabited lower slopes. (4) As angiosperms requiring warmer climatic conditions moved out onto the lowlands in response to climatic change, other angiosperms that formerly inhabited higher elevations experienced a corresponding down-slope migration. (5) Many deciduous angiosperms could migrate northward, perhaps along upland routes, whereas only a few of the warmer-climate types could migrate into the cooler northern regions. (6) As climates continued to cool, angiosperms that required warmer conditions began to be eliminated from mid-latitude lowland forests, and were replaced through down-slope migrations of those requiring more temperate climates. (7) Finally, and simultaneously, in lowlands of Arctic regions, the warmer-climate gymnosperms, ferns, and a few angiosperms were eliminated by the same climatic factors that had affected the vegetation of mid-latitude regions, and were replaced by those angiosperms that previously had migrated northward along upland routes.

Fig. 4. Map of Northern Hemisphere showing Eocene isoflors and present January isotherms, from Chaney (1940). A-Subtropical isoflor. B-Temperate isoflor. C-Cool-temperate isoflor.

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POSSIBLE FACTORS OF CLIMATIC CHANGE

Polar shift:
Much paleomagnetic evidence has accumulated in recent years that seems to indicate a shift of the earth's magnetic pole from lower to higher latitudes. It also has been assumed that the axial (rotational) pole has moved with the magnetic pole. Runcorn (1955, p. 283), on the basis of paleomagnetic data, stated:

If the coincidence of the magnetic axis with the axis of the earth's rotation is taken as a fundamental hypothesis, the poles of the earth cannot have been in the same relative position with respect to the land masses as at present. If there has been no relative movement between the land masses then there must have been movement of the whole crust at least and more plausibly the whole earth relative to the axis of rotation in space. This is known as the hypothesis of polar wandering.

The recent attention that has been placed on these problems is illustrated in such symposium volumes as "Continental Drift," edited by S. K. Runcorn (1962), "Polar Wandering and Continental

Table II. ANGIOSPERM RECORD OF HIGH-LATITUDE NORTH AMERICA (ABOUT 70° N.)

Table III. ANGIOSPERM RECORD OF MID-LATITUDE NORTH AMERICA (ABOUT 40° N.)

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Drift," edited by A. C. Munyan (1963), and "A Symposium on Continental Drift" by the Royal Society, London (1965), reviewed in Science by Gilluly (1966).

Cox and Doell (1960, p. 753) note that, "The Jurassic virtual geomagnetic poles calculated for all continents show considerable scatter, and a relatively large number are not significantly different from the present geographic pole." Their data for the Cretaceous show a paucity of evidence for this period, but the scattering seems to be centered around the Alaska-Bering Sea area. They point out (p. 762) that paleomagnetic evidence from the Northern Hemisphere is not adequate to permit conclusive statements concerning polar wandering during Jurassic and later geologic time.

Analyses of later Mesozoic floras and floral sequences across the continents of the Northern Hemisphere, presented on preceding pages, suggest a vegetational and inferred climatic zonation little different from that of today. As these are fundamentally controlled by latitude, the evidence suggests an essentially stable axial position at least since the beginning of the Jurassic. If the various floral sequences known from middle and higher latitudes of Northern Hemisphere continents are considered, and if it is noted that all contemporaneous sequences indicate the same type of climatic change, it is apparent that, if polar wandering did occur, it had no noticeable influence on vegetational and climatic zonations of the Northern Hemisphere. On the contrary, if polar wandering had occurr d, vegetational sequences (at least in the vicinity of the polar path) would be expected to show a cooling trend in one place as the pole approached, and a warming trend in another as the pole receded. This is not evident in the rich floral records.

Continental drift:
If one can infer, therefore, that the axis of rotation essentially remained stable since at least the middle of the Mesozoic, the possibility of continental movements then must be considered as an explanation of the changes in continental climates.

The idea of slippage of the entire crust, with continents "frozen" in their present relative positions, must be rejected for the same reasons that apparently eliminate appreciable shifting of axial poles during the same time interval. If the crust were to slip northward on one side of the earth to account for a cooling trend there, the opposite side would shift southward with fossil evidence indicating a contemporaneous warming trend.

The Mesozoic and Cenozoic records from different latitudes, suggesting floral zonations generally paralleling the latitudinally controlled zonations of today, support the view that Northern Hemisphere land masses had the same north-south alignments during the Mesozoic that they have at present. This apparently rules out significant continental rotations as causal factors of climatic change. If it is assumed that changes in continental climates result mainly from latitudinal movements of land masses (and polar wandering, if it took place, seems to have had little noticeable effect), then one would need to impose the so-called "yo-yo" concept of repeated north-south continental movements to explain the climatic changes since the beginning of the Jurassic. The large land mass of Eurasia, the smaller North American continent, and the relatively small mass of Greenland thus would need to move north and south in unison to explain the similarity of contemporaneous vegetation at equivalent latitudes, and at the same time retain essentially the same north-south alignment that occurs today.

The very close floristic similarity between Mesozoic vegetation of northeastern Eurasia and northwestern North America suggests that these land areas were in close proximity in the Mesozoic as they are today. There is no evidence that the floras of the two regions gradually became more alike during the later Mesozoic, as would be expected if the North American continent had drifted westward along latitudinal lines.

Northrop and Meyerhoff (1963) have summarized the various hypotheses relating to polar and continental movements, and suggest that the known factual data may be best explained by an essential stability of poles and of continental masses. Their Figure 2 (p. 583), representing an "Isopole Map" for geologic time, seems to be strong evidence favoring the concept that the present north polar region gradually evolved into a general center of earth magnetism since the Precambrian. Their compiled data for Jurassic-Cretaceous time conform reasonably well with evidence which has been presented on preceding pages of this paper. The isoflor (and inferred isotherm) lines presented by Vakhrameev (1964)

End_Page 860------------------------------

for the Eurasian continent, and zonations that appear probable for North America (this paper), indicate the existence of an axial pole in the general area of its present location, and in the general area of Northrop and Meyerhoff's geomagnetic center, during the Jurassic-Cretaceous interval. Evidence for the Cenozoic, presented by Chaney (1940) and by Durham (1952), indicates that floras and marine faunas (and interpreted temperature zonations) are consistent with polar and continental positions of the present for the interval since the Cretaceous.

Axelrod (1963) concluded that floral evidence from late Paleozoic to late Mesozoic time suggested stable rather than drifting continents. Hamilton (1964) disagreed, emphasizing chiefly the evidence from "Gondwana" continents and the interval of the later Paleozoic. Axelrod's Triassic evidence for North America seems to indicate a continental stability at least since the beginning of the Mesozoic.

The abundant Cretaceous evidence for Northern Hemisphere continents, and Vakhrameev's evidence for the Jurassic-Cretaceous of Eurasia, strongly imply a polar and Northern Hemisphere continental stability since the beginning of the Jurassic. The limits of the present study, however, do not permit an analysis of all of geologic time. The present analysis is intended, instead, to determine if the stability of continents and poles recognized for the Cenozoic also can be recognized for Mesozoic time. It would appear that, if continental drift did occur, it took place before the Jurassic, and probably before the Mesozoic (Axelrod, 1963).

Change in angle of axial inclination:
If the evidence does not favor wandering poles, drifting continents, or a slippage of the crust as causal factors of climatic changes on Northern Hemisphere continents, the possibility must be considered that the angle of inclination of the earth's axis relative to the plane of its orbit has changed through time. The angle of inclination is now about 23½° from the vertical, permitting long intervals of winter darkness north of the Arctic Circle.

The claim often is made that few plants can become specially adapted to such prolonged periods of darkness. However, hundreds of species of flowering plants now grow in Arctic Alaska under such conditions (Wiggins and Thomas, 1962). Some of these are woody perennials that, together with many herbaceous species, have a distributional range southward to upland elevations in middle latitudes (or, this might be expressed conversely as an upland distribution in middle latitudes, with a northward extension into regions of winter darkness). Their geographic distributions seem to be controlled mainly by factors of temperature and severity of winters (length of growing seasons), rather than by prolonged winter darkness or by the angle at which the sun's rays strike the area. It may be noted al o that plants in mountainous regions of western America commonly are covered by snow drifts through much of the year, yet survive and flourish when they are again exposed to the sun. It might be concluded, therefore, that other woody plants also would invade the Arctic if world climates were to return to their more normal warm conditions.

When the probability is considered that deciduous woody dicotyledons evolved in higher elevations of lower latitudes, and later migrated into high-latitude regions, it is apparent that their deciduous habit would already have been developed prior to their poleward migration (see also Axelrod, 1959, 1966). When forests containing these deciduous plants extended northward to occupy coastal-plain sites of higher latitudes, the additional factor of prolonged winter darkness probably would have had little further effect during a season of the year when these plants already would be in a dormant condition. It would seem, therefore, that no change in axial inclination need be required for deciduous forests to exist north of the "Arctic Circle" during times of warmer world climates.

Change in world climates:
Evidence from marine invertebrates and plants indicates that later Mesozoic climates of high northern latitudes were different considerably from those which now prevail in Arctic regions. If continental movements or an unstable axis of rotation can be eliminated as the ultimate cause, then it must be concluded that the climatic variations of the past were induced by some other terrestrial or extraterrestrial factors that influenced climates of the entire world.

An explanation that commonly is given involves varying degrees of volcanic activity, providing increased or decreased amounts of carbon dioxide or dust to the earth's atmosphere. However,

End_Page 861------------------------------

there seems to have been major volcanism in some part of the world through most of geologic time, and intervals when activity was most pronounced do not seem to coincide with intervals of either warm or cold climates. To illustrate: in the Soviet Arctic volcanic ash or bentonite is abundant and widespread through Jurassic and Cretaceous deposits (Sachs and Strelkov, 1961), yet paleobotanical evidence indicates that climates changed from cooler to warmer to cooler during this interval. In northern Alaska volcanic material is scarce or absent in sediments older than Cenomanian and is a common constituent in rocks of later Cretaceous age. Yet floral records from northern Alaska indicate that climates were becoming cooler from Albian time to the end of the Mesozoic. Volcanic activity has een common and widespread through much of the Cenozoic: during the early Tertiary when climates were warm, the middle Tertiary when climates were more temperate, and the later Cenozoic when climates were cooler. Such evidence does not suggest a close relation between volcanic activity and significant changes in world climates.

It would appear, then, that changes in climates of the world have probably resulted from: (1) changes in configuration of the earth's surface resulting from orogenic or epeirogenic activity, and the presence or absence of extensive epicontinental seas; (2) changes in the amount of energy received from the sun; or (3) some other factor as, for example, long-range shifts in the position of high-atmosphere currents like the "jet stream." Any one or a combination of these factors might result in such long-ranged climatic changes as have been observed in geologic time. It should be remembered that world climates seem to have been much warmer than now through most of geologic time, and the glacial or near-glacial climates of the Quaternary are the exception rather than the rule. An apprecia le warming or cooling of world climates could be expected to result in significant changes in vegetation of Arctic regions, without the advent of continental or axial instability.

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Detterman, R. L., R. S. Bickel, and G. Gryc, 1963, Geology of the Chandler River region, Alaska: U. S. Geol. Survey Prof. Paper 303-E, p. 223-324.

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Acknowledgments:

(2) Department of Geology and Geography, College of Mines, University of Idaho.

All field projects, 1956 to 1966, were sponsored by the Arctic Institute of North America and the Office of Naval Research, Department of the Navy. Logistic support in the field was provided by Max C. Brewer and staff of the Arctic Research Laboratory, Point Barrow. Many geologists of the U. S. Geological Survey, especially of the Alaskan Geology Branch, contributed much information and advice in connection with these field studies. D. I. Axelrod and A. A. Meyerhoff are acknowledged with gratitude for their critical review of the original manuscript. British Petroleum and Phillips Petroleum Companies offered the use of their field helicopters in 1965 for access to otherwise inaccessible areas or in time of emergency. Field student assistants at various times were Bruce Arnett, Dick Bi gerstaff, Don Hartman, Don Jennings, Howard Schorn, and Ibrahim Shah.

Copyright 1997 American Association of Petroleum Geologists

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