The Yukon-Koyukuk Region
|Introduction||Geology||Hollick Localities||Spicer Localities||Locality Relationships||Wider Correlations|
Scott and Smiley (1979) were reluctant to rely on Hollick's (1930) work and suspected that the collections Hollick used may have come from different stratigraphic levels. This attitude is particularly understandable in view of Hollick's confused systematics and the fact that his material was collected by a number of workers at different times. Subsequent re-examination of Hollick's localities by RAS, nevertheless, confirms that the overall associations between forms at particular localities are as he reported, except that the facies control of assemblage composition is stronger than was previously thought.
While it is true that the allocation of different specimens to the same form is to some extant subject to the personal philosophy of the individual paleobotanist, by adopting rigorous criteria and using as many characters as are available in the specimens, the repeatability (accuracy) of determinations is likely to be improved. A strict approach necessarily reduces the number of forms common to different localities, and therefore restricts their biostratigraphic usefulness, but it does improve the value of multiple specimen assignments to the same form when they occur.
The table below shows the occurrence of the fossils at Spicer's localities. The paucity of magnoliid-like forms in the coarser-grained facies of locality 11561 and their abundance, together with various gymnosperms, in the finer-grained sediments of locality 11556 is immediately obvious. Abundant conifer impressions were often found in dark paludal carbonaceous shales. Unfortunately preservation was so poor and the plant remains so densely packed that identification was not possible and they are not figured here.
Correlation chart of the fossil forms and their occurrences (indicated by a '+') in the localities along part of the Lower Yukon River, Alaska as shown in the localities map.
|FORMS||Fine Sand||Sand||Gray siltstone||
Gray micaceous shales
|Shales||Gray Siltstone||Siderites||Micaceous fine sand||Sand||Micaceous siltstone||Fine to medium sand||Micaceous fine sand||Shales and sand||Sand|
At other localities, particularly 11561, the sediments that yield the most abundant and varied platanoid forms are the fine- to medium-grained micaceous fluvial sandstones or the siltstone lenses associated with them. Sorting of both the inorganic sediments and the plant debris is poor and often there is considerable evidence for rapid deposition. Impressions of branches are common as is the presence of large scale cross-bedding. In the siltstone lenses leaves are often densely packed and moderately well bedded.
Locality 11556 was particularly rich in plant remains. It yielded a variety of forms and had the largest number of forms in common with other localities. The richness of the assemblages may, in part, be because the matrix varies in grain size. While the sediment is predominantly a calcareous iron-rich claystone, coarser siltstones yielding the larger platanoid forms (HAPLTL29 and HAPLD27) are found. Even the finer-grained sediments often exhibit signs of being rapidly deposited because bedding of both the inorganic matrix and the plant remains is frequently poor. Single blocks of matrix 0.3 m or so in diameter show a range of bedding development from finely laminated fine-grained sediment to coarser siltstone which has no discernible bedding, and which yields poorly sorted plant debris entombed at various orientations with respect to the laminated sediments.
Unfortunately the sediments cannot be studied in situ as all of the specimens were collected from slumped material at the foot of the slide indicated by Martin in Hollick (1930) (Fig. 4, p. 24). The localities are restricted to the river bank and dense vegetation prevented any investigation of possible exposures up the small valley which Martin in Hollick (1930) suggests was the source of the material. Although not in place, the suite of fossils is considered to represent a single assemblage because characteristics of deposition and lithology, except grain size, are more or less the same between the different specimen-yielding blocks.
Hollick (1930) was also of the opinion that his collections from this area (Lots 2962, 3252 and 3536) represented the same assemblages. Furthermore he recognized that ferns and gymnosperms were more abundant here and suggested that the collections represented a " *** horizon slightly older than any of the others, or *** an environmental phase of the same period of deposition" (Hollick, 1930) (p. 3).
Locality 11556 is interpreted as representing a body of relatively quiet water perhaps not directly connected to the main river channel but which was subject to periodic innudation by flood waters. It therefore predominantly reflects the backswamp community and the platanoid hamamelid-like leaf forms were either growing immediately around this depositional site or were washed or blown in from very close by. The fact that platanoid leaves occur at this locality cannot be interpreted as necessarily meaning they were transported to the site from elsewhere. It would be surprising if the facies associations were mutually exclusive as species separations between river margin and backswamp communities are not absolute even in floodplain environments today, and some species overlap might well have occurred in the Cretaceous communities.
The mixed nature of the assemblage at locality 11556 provides the key for understanding the contemporaneous nature of the assemblages laid down in the swamp/lacustrine and fluvial environments, because it allows the relative ages of the various localities to be compared. Of the eleven forms found in more than one locality, six are platanoid hamamelid-like leaves, one is a RAPE leaf, one a gymnosperm, one a fern, and two are magnoliid-like leaves.
The diagram below illustrates the relationship between localities based on shared forms. The solid lines indicate forms represented by what are considered to be identical specimens from more than one locality. The dotted line illustrates the shared occurrence of fragments of similar platanoid leaf material but which cannot be referred to any of the more complete specimens. Localities 11550, 11551, 11552, 11554, 11556, 11559, 11561, 11571 and 11572 are all tied together suggesting that they are probably all of approximately the same age. Of the remaining localities, 11557 and 11558 share a single platanoid hamamelid-like form and 11549 and 11553 have no forms in common with any other localities. Locality 11549, while being rich in plant remains, yielded mostly highly fragmented and poorly preserved materials. Only one reasonably intact, yet still poorly preserved, specimen was recovered. Similarly locality 11553 only yielded one specimen and therefore it is not significant that shared forms with other localities are lacking. That specimen USGS 11549.1 is a platanoid hamamelid found in micaceous siltstone and USGS 11553.1 is a magnoliid-like leaf from a dark gray shale is however consistent with the assemblage/facies relationships of the other localities.
Diagram showing relationships between Yukon-Koyukuk localities based on shared forms. The dark lines indicate forms represented by identical specimens from more than one locality. The light lines indicate the shared occurrence of fragments of similar platanoid leaf material but which cannot be referred to any of the more complete specimens.
On the basis of shared forms there seems to be no significant difference in age between assemblages from the localities representing rocks originally mapped by Martin (in Hollick, 1930) as belonging to the Melozi Formation and those belonging to the Kaltag Formation. The large numbers of forms not common to the various localities are of no biostratigraphic importance as far as comparative age determination within the Yukon-Koyukuk Province are concerned, because they represent species separation in the original contemporaneous plant communities.
Locality 11566 probably represents the Nulato Formation (Martin, 1926) in Hollick (1930) and is the only site described here within the lower unit of the interior facies (Kil) of Patton and Bickel (1956). Only one specimen was collected and this was from an approximately 1 m diameter fallen block at the foot of the talus slope, well above the normal river level. Part of a frond of Pseudocyas unjiga (Dawson) Berry is represented in a fine micaceous sandstone. This species occurs most frequently in the lower Upper Cretaceous and in North America and is confined in the stratigraphic range upper Albian to Senonian. It has also been recovered from similar sediments on Nelson Island, Western Alaska.