The Yukon-Koyukuk Region

Introduction	Geology	Hollick Localities	Spicer Localities	Locality Relationships	Correlation	Leaf architecture

 

Correlations Based on Leaf Architecture

Whereas the Potomac Group cannot provide reliable correlations with the Yukon-Koyukuk Province assemblages on the basis of shared forms, it is of critical importance in the context of comparative leaf architectural studies largely because of the unique pioneering study of Doyle and Hickey (1976). The good palynological control, which is missing in many of the Alaskan successions, provides a rigorous stratigraphic framework into which temporal changes in leaf architecture may be placed.  In their studies of plant fossil assemblages from the Patuxent Formation, Arundel Clay, Patapsco, and so-called 'Maryland Raritan' (Elk Neck locality of Patapsco Formation of Wolfe and Pakiser; 1971) Formations, Doyle and Hickey (1976) document sequential changes in the increasing degrees of vein organization, including the differentiation of veins into discreet orders, and consistency of vein courses.

In the lower Albian(?) Fredericksburg and Drewrys Bluff localities, Virginia, (Zone I) entire margined pinnately veined forms occur with poorly organized venation (Text Figure 6b).  Anastomosing veins enclose elongated rhombic areas of lamina (for example Rogersia angustifolia Fontaine, Celastrophyllum latifolium Fontaine; Text Figures 6a and 6c).

In middle Subzone IIB (middle to upper Albian) palmately veined forms occur for the first time.  In view of the range of morphology found in later platanoid forms it is perhaps significant that palmately veined peltate and lobate-cordate-ovate leaves intergrade with each other with respect to shape, venation, and margin characters.  The palmately veined forms also show more regularity in their vein courses than Zone I leaves and such forms as Menispermites virginiensis Fontaine (Text Figure 6d) possess tertiary veins comprising an irregular reticulum, but with a tendency to form convex transverse bridges between the pectinal veins.

The middle of Subzone IIB also yields leaves with doubly convex glandular serrations as well as both pinnately and palmately lobed forms.  Of interest with respect to later platanoids is the occurrence of palmately lobed forms (Araliaephyllum obtusilobum Fontaine; Text Figure 6e) with palinactinodromous venation and a tendency for the slightly irregularly spaced secondary veins to be looped.  The tertiary veins at the leaf base are irregularly percurrent becoming weak and irregular above.  This type of leaf is apparently restricted to coarser sediments and shares both magnoliid and platanoid characters.

Upper Subzone IIB (upper Albian) sees a possible elaboration of the cordate-reniform complex.  Leaves such as Menispermites potomacensis Berry (Text Figure 6f) have an ovate or shallowly lobate shape, entire margins, and approximately six radiating veins of primary order which divide and loop near the margin.  Other leaves such as Populophyllum reniforme Fontaine (Text Figure 6g) from upper Subzone IIB also have radiating major veins which irregularly divide and rejoin forming poorly organized loops but are irregularly lobed.  As with RAPE11, RAPE12, and RAPE13 the venation shows a tendency to be bilaterally arranged about a single midvein but overall the vein organization is poor compared to the Yukon specimens.  That the Menispermites-type leaves of the Yukon-Koyukuk Province are evolved from the same complex as the Potomac forms cannot, as yet, be discounted.

Truly pinnately compound leaves first appear in upper Subzone IIB of the Potomac Group.  These Sapindopsis-type leaves (Text Figure 6h) have leaflets with pinnate festooned brochidodromous venation and secondary and tertiary vein orders are readily distinguishable (Text Figure 6i).  With the possible exception of MA59 pinnately compound leaves are missing from the Yukon sequences.

Palmately lobed leaves are also present in upper Subzone IIB (middle to upper Albian) and become dominant in Subzone IIC (uppermost Albian to lower Cenomanian).  Confined to the coarser facies they are typified by 'Sassafras' potomacensis Berry (Text Figure 6j).  The looped secondary venation of these leaves is far less regular than anything seen in the Yukon material and the tertiary veins are weak, closely spaced, and irregularly and obliquely percurrent.  This form continues into Subzone IIC by which time it has one of the most poorly organized venation systems of any of the lobed 'platanoids'.

The Subzone IIC 'platanoids' possess strong, rigidly organized, tertiary veins perpendicularly crossing broad intercostal areas, with subparallel transverse quaternary veins.  The secondary veins of such leaves as Araliopsoides cretacea (Newberry) Berry (Text Figure 6k) are still brochidodromous and the tertiary veins less regular than seen in all but one of the Yukon platanoids.  This particular form has a decurrent base although peltate bases are also common.  Protophyllum multinerve Lesquereux (Text Figure 6l) (Subzone IIC - ? Zone III), which Doyle and Hickey (1976) consider a possible unlobed member of the same complex, does, however, display third and fourth order venation as well organized as any in the Yukon-Koyukuk Province.

The level of vein organization seen in the specimens from the Yukon-Koyukuk Basin is not achieved in the Potomac Group until the lowermost part of the Cenomanian.  Clear differentiation of vein orders is seen in most of the Alaskan platanoid leaves, and is even displayed in the well preserved Menispermites-type leaves (RAPE11, RAPE12, and RAPE13) which posses lower vein orders with somewhat irregular vein courses.  Furthermore the lack of any palmately veined forms with poorly organized tertiary and higher order venation must place the Yukon assemblages developmentally above even the Potomac Zone III assemblages.

The platanoid leaf from the Yukon-Koyukuk Province with the least number of advanced features is HAPL27.  Here the pectinal abmedial veins are irregularly looped, inter-abmedial veins are present, and the tertiary venation is somewhat irregular.  Fourth order veins display minimal tendency to run percurrently between the tertiary veins and are poorly differentiated from higher order veins.  The teeth are apparently non-glandular with the medial vein terminating, without thickening, at the tooth apex.

The similarity in architecture between form HAPLD27 and Araliopsoides cretacea suggests that HAPLD27 is derived from the Araliopsoides complex.  The level of vein organization in HAPLD27 is slightly lower than that seen in A. cretacea and HAPLD27 could easily be a relictual form.

A significant differences between the forms is the development of semicraspedodromous veins supplying the apparently non-glandular teeth of HAPLD27 in contrast to the entire margins of Araliopsoides Berry.  Entire margined palmately veined leaves are absent in the Yukon assemblages; a fact which may be associated with cooler climatic conditions.  The tertiary veins running between the basal superior secondary veins and the pectinal veins are distinct and convex percurrent in HAPLD27, whereas in Araliopsoides they have a tendency to be irregular or sinuous in course and irregularly branched.  On the other hand the tertiary veins running between the abmedial veins of HAPLD27 are poorly organized compared to the regularly spaced, seldom branched, percurrent tertiary veins of Araliopsoides.  The pectinal veins of HAPLD27 are weak and there is no evidence of lobing.

Lobed leaves from the Yukon-Koyukuk Province assemblages that are also clearly similar to those of the Araliopsoides complex are represented by form HAPLD2. In this form both the secondary and abmedial veins are craspedodromous and one of the abmedial veins is strengthened in a return to the palinactinodromous condition seen in Araliaephyllum obtusilobum of the Potomac middle Subzone IIB.

While clearly similar in gross venation, specimens USGS 11558.4 and USGS 11557.2 (HAPLD2) differ in a number of respects.  The teeth of USGS 11558.4 are considerably coarser then those of USGS 11557.2 and the tertiary veins are more irregular in course and branching.  As far as can be determined the poorly preserved specimen USGS 11558.1 is intermediate between these two leaves.  The significance of these apparently more primitive characters of USGS 11558.4 cannot yet be evaluated, but in view of the usual variation found in such leaves they have here been included, perhaps erroneously, in a single form.

By the beginning of the Cenomanian the finer-grained facies of the Potomac Group yield predominantly simple, pinnately veined, second rank (sensu Hickey, 1971) leaves with poorly organized but broadly camptodrome venation. These leaves evidently possessed venation systems even less well organized than those of similar type found in the equivalent facies of locality 11556, which again indicates that the Yukon fossils are no older than early Cenomanian even from this locality which Hollick (1930) suggested might pre-date the others.

 

 

Text Figure 6.  Venation diagrams of selected leaf specimens from the Potomac Group (redrawn from Doyle and Hickey, 1976):

a) Rogersia angustifolia Fontaine (USNM 192339) from Zone I (X1).
b) Detail of marginal and intercostal venation of a fragment of a large leaf of Ficophyllum Fontaine (USNM 192353) from Zone I (X2).
c) Fragment of a leaf belonging to the Celastrophyllum latifolium Fontaine complex (USNM 31814) from Zone I (X1).
d) A fragment of a large peltate leaf of Menispermites virginiensis Fontaine (USNM 3248) from middle Subzone IIB (X 0.75).
e) A middle Subzone IIB example of a member of the Araliaephyllum obtusilobum Fontaine complex (USNM 201916) (X1).
f) Menispermites potomacensis Berry (USNM 201918A) from facies equivalent to upper Subzone IIB (X1).
g) Fragment of a lobed leaf similar to some members of the Populophyllum reniforme Fontaine complex (USNM 201917) from facies equivalent to upper Subzone IIB (X1).
h) Reconstruction of Sapindopsis Fontaine from upper Subzone IIB (X1).
i) Detail of venation of Sapindopsis (USNM 201919) from upper Subzone IIB (X2).
j) 'Sassafras' potomacensis Berry (USNM 201920) from upper Subzone IIB (X1).
k) Araliopsoides cretacea (Newberry) Berry (USNM 201921A) from Subzone IIC -? Zone III (X 0.75).
l) Fragment of leaf of Protophyllum multinerve Lesquereux (USNM 201922) from Subzone IIC - ? Zone III (X1).