Download Excursion #5

Field trip No 5
Sedimentology and Palaeontology of Devonian
Zvārte rock - cross-stratified sandstone and clayey deposits of the Gauja Formation (Upper
Devonian) and light fine-grained sandstone of the Amata Formation (Upper Devonian); many
fish fossils in the top part of the sandstone section.
Lode placoderm deposit - located at the Liepa (Lode) clay deposit, Lode Formation, Upper
Devonian. All types of Devonian deposits in Liepa quarry are enriched in fossil remains. Studies
of V. Kuršs, E. Lukševičs, I. Upeniece, I. Zupiņš and other specialists have shown that likely it
was caused by nutriment-rich environment in the deltaic zone, which existed along the northern
margin of the Devonian basin including an area of Liepa, as well as by good conditions for
preservation of fossils. Particularly rich fossil assemblages are present in the lenses of grey fine
clays, which accumulated in big slump depressions, more than 100 m wide and several tens of
metres deep. Fine grained nature of sediments, as well as a very calm and anoxic sedimentary
environment probably were the most important factors, which favoured accumulation and good
preservation of fossils in these clays. Thanks to the detailed studies by V. Kuršs, L. Lyarskaja, E.
Lukševičs, I. Upeniece, J. Upenieks and I. Zupiņš, there are found well-preserved placoderms,
acanthodians, coelacanth etc. In places large accumulations of exceptionally well-preserved
complete bodies of fishes, mostly Asterolepis ornata and Laccognathus panderi, have been
found. In some accumulations of fish remains well-preserved small (1-4 cm long) bodies of
juvenile fishes occur. Invertebrates and plants, which probably served as a nutriment for fishes,
quite often are present in grey fine clay lenses. Fossil parasitic flatworms (Platyhelminthes) have
been recently found by I. Upeniece, and this is the first discovery of parasitic worms in so old
fossil record. Certainly, Liepa quarry still keeps surprises for palaeontologists in future.
Ērģeles cliff - located at Priekuļi Parish, on the left bank of the Gauja. It is one of most typical
outcrops of the Gauja Formation (Upper Devonian) in Baltic, stratotype of lower part of the
Gauja Formation. There is exposed cross stratified, yellowish sandstone. Some metres above the
water level along almost all the outcrop wall exposes the erosional surface, in places covered by
clay pebbles, more rarely phosphate pebbles. Cross-stratification is well-expressed, and it
indicates the dominant role of currents in the Devonian sea. Often the bedding is crossed by
reddish and brown belts of iron compounds formed by groundwater, which penetrated though the
porous sandstone, washed out iron compounds and redeposited them in other places.
The name of outcrop (Ērģeles = organ) comes from story that in opposite bank of the Gauja
River a boy has shepherded a drove. When he called animals, an echo sounded like an organ.
During Soviet time, to stop to use the name "organ" (ērģeles) the cliff has been called "Ērgļu
(Eagles) cliff".
Ķūķu cliff - stratotype of the boundary between the Gauja and Amata formations (Upper
Devonian). In upper part sandstone and clay of the Amata Formation; below them - siltstone,
clay and sandstone of the Gauja Formation. The lowermost part of outcrop reaches directly the
Gauja River. Devonian fish fossils are well-studied in the outcrop.
Sietiņiezis (Fine-sieve rock) - located at Vaidava Parish, on the left bank of the Gauja River. It is
the largest outcrop of bright white sandstone in Latvia. By geological age it belongs to the Sietiņi
Formation (Upper Devonian), and it is the stratotype (most characteristic outcrop) of this
formation. The precipice is up to 15 m high, but the total thickness of exposed deposits reaches
34 m, because the bedding is slightly inclined. In splendid and unusually white outcrop wall the
bedding of Devonian sandstone is well seen (marked by dark laminae of the heavy minerals),
and it gives evidence about sedimentary environment in the Devonian sea before c.a. 380 million
years. Sietiņiezis has been one of the best study objects for well-known Devonian researcher
Professor Visvaldis Kuršs. Most typical sedimentary structure in the outcrop, like in other
exposures of the Devonian sandstone, is trough cross-stratification, which gives evidence about
transporting and accumulation of sand in subaqueous dunes due to current action. In places, due
to slump processes, small depressions had been formed in the sea bed. Current velocity
decreased above the depressions, which caused rapid accumulation of transported sand and
created a specific "double" cross-stratification. Deformed (folded) bedding typical for slump
depressions and "double" cross-stratification indicating their filling by sand is visible in lower
part of northern flank of
Sietiņiezis, 1 m above the Gauja water level. In lower part of the Sietiņiezis precipice sandstone
contains many bluish-grey siltstone pebbles, and its beds became thicker to south. It is supposed
that in southern direction the Devonian sea was deeper, and more sand together with pebbles has
filled it. The upper part of the Sietiņiezis, placed in forest more distant from the Gauja River,
does not show "double" cross-stratification, siltstone pebbles and laminae of heavy minerals, but
there are many deformations of cross-laminae, mostly small folds and convex laminae, which
have formed, when water-filled sand slumped on the Devonian sea bed. In several places the
modern geological processed have affected the Sietiņiezis outcrop. There is large suffosion (sand
washing by water) funnel, several caves (largest is Devil's cave, ~10 m long) and niches, as
well as the large arch previously crossed by a pathway, but now fallen down. One spectacular
sandstone remnant has been called Vella papēdis (Devil's heel).