252 
proving the extent of the overthrusting. 
Pressure for more oil and gas discovery 
will provide the incentive for the 
drilling of the deep holes necessary 
to test the formations below the faults. 
(2) Plains of Alaska and western 
Canada, and eastern Mexico and Guate- 
mala.—These two areas, which appear 
to be the northern and southern ex- 
tremities of the western plains regions 
of the United States, much of which 
has been so productive of oil and gas, 
cover an area of over a million square 
miles in Alaska, Canada, and northern 
United States, and 300,000 square 
miles in Mexico and Guatemala. 
They are considered together be- 
cause they have many common char- 
acteristics significant in the geology of 
petroleum. Both contain large vol- 
umes of sediments, Both also contain 
numerous and widespread evidences 
of petroleum in the form of seepages, 
asphalt and tar deposits, as well as 
proven oil and gas fields. There can 
be no question as to abundant source 
rocks. 
The geologic section in the western 
Canadian plains contains five regional 
unconformities, and the Mexican sec- 
tion contains as many or more. One 
of the important functions of uncon- 
formities is to mark the truncation of 
porous formations below the uncon- 
formity. Likewise, the overlapping 
formations above the plane of the un- 
conformity often contain porous mem- 
bers or lenses. Such stratigraphic 
phenomena close to unconformites 
account for a substantial percentage of 
the world’s oil fields. Probably of 
equal importance to the petroleum 
geologist is the masking effect of un- 
conformities—they conceal the under- 
lying geology. ‘The presence of an 
unconformity in the geologic section is 
notice to the exploration geologist that 
a new and unknown set of conditions, 
structural as well as stratigraphic, may 
be expected to exist below. In many 
provinces, drilling through unconform- 
ities has revealed new and unsuspected 
deeper formations which caused both 
geological and geophysical upsets. 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
In addition to the unconformities, 
sections of rocks in both areas show 
numerous changes in facies; lateral 
variations in porosity, limestone reefs, 
and interbedded continental and ma- 
rine sediments. The combination of 
the unconformities and changes in 
facies gives to both areas innumerable 
possibilities for stratigraphic traps of 
every conceivable variety. Local 
deformation in an area characterized 
by such stratigraphic variations need 
not be strong. Gentle folds, structural 
terraces, and other minor structures 
can easily become effective traps for 
large oil pools especially when com- 
bined with lateral changes in porosity 
and permeability. 
The average thickness of sediments 
in these two areas which might carry 
petroleum is on the order of 2 miles, 
which means a total volume of sedi- 
ments of over 2% million cubic miles— 
a truly large volume of prospective 
material which is today essentially 
unexplored! It is inconceivable that 
such a large volume of rocks, in which 
every prerequisite of a petroleum prov- 
ince is richly developed, should not 
be the site of hundreds upon hundreds 
of oil fields yet to be discovered. 
Deposits such as the Athabaska oil 
sands of Canada or the Golden Lane 
and Poza Rica fields in Mexico give a 
measure of the possible size of some of 
these undiscovered fields. Not until 
thousands of additional test wells have 
been drilled throughout both regions 
can we say that the exploration of these 
areas has been completed. 
(3) Southeastern United States.—The 
dome structure in contours might be 
called the symbol of the petroleum 
geologist of the 1920’s—it was prac- 
tically the sole objective in his search 
for traps. While a dome structure is 
still as desirable a guide to an oil pool 
as ever, it is gradually being learned 
that stratigraphic variations are prob- 
ably equally as important as folding 
and faulting in the formation of traps. 
One of the greatest oil fields in the 
world, the East Texas field, has a cross 
section which is symbolic of the chang- 
