Geoscience Reference
In-Depth Information
methods of time-series analysis (e.g., Trauth et al. 2000, Marwan et al. 2003).
h e El Paso section in the Quebrada de Cafayate contains well-developed
annual layers in most parts of the profile (Fig. 8.7). h e base of each of these
mixed clastic and biogenic varves consists of reddish silt and clay, with a
sharp lower boundary. Towards the top of the varves, reddish clay and silt are
gradually replaced by light-brown to greenish clay. h e change from reddish
hues correlates with a slight decrease in grain size. h is clastic portion of
the varves is capped by a thin layer of pure white diatomite. Diatomites are
sediments comprised mainly of porous siliceous skeletons of single-cell
algae, i.e. diatoms. h is internal structure of the laminae is typical of annual-
layered sediments. h e recurrence of these layers and the distribution of
diatoms, together with the sediment coloration and provenance, all provide
additional evidence that rhythmic sedimentation in this region was controlled
by a well-dei ned annual cycle. h e provenance of the sediments contained
in the varved layers can be traced using index minerals characteristic of the
various possible source areas within the catchment. A comparison of the
mineral assemblages in the sediments with those of potential source rocks
within the catchment area indicates that Fe-rich Tertiary sedimentary rocks
Fig. 8.7
Photograph of varved lake sediments from the Quebrada de Cafayate in the Santa
Maria Basin, with cyclic occurrences of intense dark-red coloration rel ecting enhanced
precipitation and sediment input during ENSO- and TAV-type periodicities (350 cm above
the base of the El Paso section). h e solid blue line denotes the course of the digitized color-
intensity transect. h e red circles note the position of the diatomite layers, representing
annual layers.
