Journal of Palaeogeography
2014, 3(4): 410
-
418
DOI: 10.3724/SP.J.1261.2014.00064
Lithofacies palaeogeography and sedimentology
Armoured mud balls as a result of ephemeral uvial
ood in a humid climate: Modern example from
Guizhou Province, South China
Gerhard H. Bachmann
Martin-Luther-Universität, Institut für Geowissenschaften, Von-Seckendorff-Platz 3, D-06120 Halle (Saale),
Halle (Saale), Germany
Abstract Armoured mud balls were observed after rainfall and a short ood in the other-
wise dry Xiaohe (small river) valley of Guanling County, Guizhou Province, South China, app-
roximately 30 km southwest of Guanling City. Armoured mud balls are most common in semi-
arid climates, but rather unusual in a humid climate as in Guizhou. A number of well-rounded
mud balls, 2
-
20 cm in diameter, were found lying on the gravel of the Xiaohe gully oor. The
mud balls consist of sticky, light brown and slightly mottled clay without carbonate content. The
surfaces of the mud balls were studded with rims of sand- or gravel-size limestone clasts, col-
lected during bedload transport, as is typical for armoured mud balls. The mud balls originated
from alluvial mudstone deposits of the valley oor and cliff that are most likely derived from the
weathering and karstication of bedrock limestones. Such mudstones seem to be especially
well suited for forming armoured mud balls. As ood events are rather common in the area,
the formation of armoured mud balls should be very frequent in the Xiaohe valley and similar
valleys nearby, giving the possibility for further and more detailed studies. To the best of our
knowledge this is the rst description of armoured mud balls in China.
Key words armoured mud balls, uvial, alluvial, mudstones, humid climate, Guizhou,
South China
1 Introduction *
Alluvial mudstones are often surprisingly resistant to
erosion, despite their unlithied and often semiconsoli-
dated character. Such is especially true for mudstones
with a high clay content because of their high cohesion.
They are most frequently eroded as smaller or larger mud
chips following desiccation and mud cracking. Other sites
of erosion are the undercut banks of streams and rivers,
where alluvial mudstones may form steep cliffs. Erosion
* Corresponding author. E-mail: gerhard.bachmann@geo.uni-
halle.de.
Received: 2014-05-28 Accepted: 2014-06-14
is enhanced, especially during ood events, by undercut-
ting, slope failure, and subsequent slumping of the cliffs.
Small and large blocks of semiconsolidated material may
then fall into the turbulent water, where they are rapidly
softened and washed away. In some cases, however, mud-
stone fragments can become embedded in silt or sand and
so preserved.
Rather unusual, however, are small or large, mostly
well-rounded mud balls that are transported like gravel or
boulders over considerable distances. The term “armoured
mud balls” was introduced by Bell (1940) for spherical
clay masses studded with pebbles. Examples are known
from the recent and fossil record (e. g. Mather et al., 2008).
Pettijohn and Potter (1964) dened armoured mud balls (or