84 SIERRA DE UMANGO 
the upper, ‚pale sandstone is concordantly overlaid with soft youn- 
ger strata, not represented in the very “intermontane“ section of the 
zone. In this section a complicated tectonic is developed, with 
folds and overthrusts, but the northern and southern continuations 
show a simple dip toward the west, a monoclinal structure. The 
age of all these layers shall be discussed later on. 
Small sunk sandstone masses in the basement. 
Limited areas of sunk or down-thrusted sandstone remnants occur 
in many parts of the Umango area. Because these occurrences 
lack every relation to the other ones of the continental cover, it is 
a difficult task to fix their real stratigraphical position. 
First is to be mentioned a somewhat narrow, but quite a long 
zone of crom-red sandstone, running along the western foot of the 
main ridge of the Sierra de Umango and with a northeasterly trend - 
crossing the same ridge in its lower, northern part. Finally the 
zone flows together with the intermontane zone (Quebrada Salto). — 
The red sandstone here is not only deeply downfaulted between 
steep walls of the crystalline basement, the strata stand also verti- 
cally in the greater part of the zone, or are laid into folds by a 
thrust movement from the west (see the tectonics). The lithological 
habit of the sandstone is very a similar one to that of the upper 
Gondwana red layers in the slopes of the Cerro Villa Union. Also 
characteristic basic lava sheets are seen in these layers. The steeply 
inclined strata are discordantly overlapped with a coarse conglome- 
rate (description of the locality on pages 113—115) and this sediment 
conceals to a considerable extent the red layers. Because the most 
interesting tectonical phenomena are visible in the vicinity of the 
Puesto Pirca (at the western foot of the mountain ridge), the zone 
may be called in the following chapters „the Pirca zone“. 
Other similar sandstone remnants in the form of longitudinal 
throws exist in the Quebrada Puesto Quemado, at the northern 
end of the Cerro Cacho and also at the very western border of the 
3 
4 
| 
j 
SVE NIER = 
OE Oe ee Te TOE 
