Christian-Albrechts-University Kiel
Germany
Department of Physical Geography
Landscape Ecology and Geoinformation
Depositional Processes, Stratigraphy and Soil Development in
Vernal Pools, Enchanted Rock, Texas
James F. Petersen, Texas State University, San Marcos (TX), USA
Rainer Duttmann, Michael Kuhwald, Nele Meyer, University of Kiel, Germany
The bedrock summit of Enchanted Rock, a 130 m granitic exfoliation dome in central Texas,
is dotted with weathering pits, some of which contain an adequate soil depth to support
vernal pools (island-gardens of vegetation). Soil analyses of two of the larger pits reveal
a significant contribution of eolian deposits to soil formation. One of the vernal pools shows an enrichment of gypsum in the deeper horizons, which points to an accumulation of wind-blown gypsum-rich dust.
study site
The soil stratigraphy from another pit, which we focus on
in this study, reveals distinct layering (see the core sample on the left). Although the soil texture varies little within
the whole profile, the content of rock fragments changes
significantly among individual layers of sediment. Sediment layers with higher proportions of grus and granitic
fragments (> 15 Vol.-%) are embedded between layers
that contain only small portions of finer gravels or grus.
We assume that the latter deposits are of eolian origin,
and the coarser sedimentary layers have been deposited
by runoff on the bare granite dome in response to rainfall
events.
Orthoimage of Enchanted Rock
Courtesy: CAPCOG, Texas
sample point
30 cm
30 cm
up
loam
39 cm
coarse gravelly
loam
down
45 cm
45 cm
up
loam
52 cm
down
gravelly loam
60 cm
up
loam
down
60 cm
cal. 14C age:
1605 +/- 30 BP
very gravelly
sandy clay loam
weathered granite
68 cm
75 cm
75 cm
Depth
Texture
Further evidence of eolian activity is suggested by the
dune-like surface form of the vernal garden, which rises up to 54 cm above the surrounding bedrock surface
and nearly 90 cm above the deepest part of the weathering pit. Surprisingly, the light colored sediment layers,
considered to be of eolian origin, also contain a noticeably high content of finely distributed organic carbon,
presumably coming from wildfires that periodically occur
during hot, dry periods, which desiccate the vegetation
in the vernal gardens. The loss of a vegetative cover
combined with strong prevailing winds on the summit
of Enchanted Rock, have contributed to deflation on the
windward side and deposition of wind blown sediment
on the lee side of the vernal garden studied here.
At this pit, AMS-14C-analyses of the deepest eolian layer
reveal a minimum radiocarbon age of 1605 +/- 30 BP
for the beginning of eolian deposition.
Sand (%)
Silt (%)
Clay (%) Carb. (%)
coarse
medium
fine
coarse
medium
fine
pH
EC
(CaCl2)
(µS/cm)
- 30 cm
loam
22.2
13.2
11.4
14.1
14.2
6.1
18.8
2.18
4.2
91
- 39 cm
loam
22.7
12.4
9.7
13.4
14.0
5.3
22.5
2.52
5.1
183
- 45 cm
loam
27.3
11.8
10.4
15.8
13.2
0.0
22.3
1.39
4.0
60
- 52 cm
loam
24.1
11.9
9.5
12.8
13.1
5.1
23.7
2.43
5.6
194
- 60 cm
loam
19.1
11.2
10.4
15.5
16.1
3.5
24.2
0.82
4.2
52
- 68 cm
loam
22.3
12.3
10.7
15.1
14.3
7.3
18.1
2.09
5.2
231
- 75 cm
clay loam
36.7
20.3
9.2
4.6
3.5
3.1
22.7
0.5
4.4
63
Terrain surface of the vernal
garden (dashed line) in m
above sealevel calculated
from LIDAR data provided
by CAPCOG, Austin, TX
Elevation (m) of the unweathered granite surface beneath
the vernal garden (dashed line).
The vernal garden covers two
deeper weathering pits (black
sections) reaching a maximum
depth of 35 cm.
3D model of the weathering
pan beneath the vernal garden