Page 15 - macneill1947

12.

conditions are fresh water gastropods, oscillatory ripple marks

and large delta formations.

Fresh-water lacustrine invertebrates have been found

at two localities, one in the northeast corner of the area near

section D-D' (plate I), the other on the easternmost anticline

in section A-A'. At the first locality are foULd detaic forset

and topset beds (figure I), and with the latter oscillation

ripple marr(s. These tvvo localities, well out in the major Mint

Canyon syncline, point to the existence of lakes at the time

these sediments were laid down. V:hether or not the lals::es were

joined remains problematical; but the possibility undoubtedly

exists.

In Mint Canyon time, the Humphreys Station region

probably was part of a valley surrounded by uplands composed

chiefly of granitic and metamorphic rocks. Rivemdeposited

the products of erosion from the uplands into local lakes.

Rapid deposition resulted in alluvial fans and deltas, and a

consequent shifting in the positions of thcr la}:es as they be-

came filled.

Modelo Formation:
11 11

Nonoonformably overlying the Mint Canyon formation is

the "Modelo" formation. Kew (op. cit. p. 67) thought that the

"Modelo" from this area was probably to be correlated with the

upper part of the "Modelo" of the type section. The formation

is assigned to the Modelo group here because of its upper