Zemes un vides zinātnes Earth and Environment Sciences - Latvijas ...

102 ADVANCES IN PALAEOICHTHYOLOGY
have been eroded. The Devonian deposits are also spread throughout the territory of
Latvia, in the Gulf of Riga and in the aquatorium of the Baltic Sea. Although not
outcropping, the Lower Devonian deposits are to be found in the whole territory of
Latvia, excluding the Viļaka tectonic rampart and the territory east of it. The Middle
Devonian rocks are widely represented throughout the whole territory of Latvia and
exposed in many sites in picturesque outcrops of the Old Red Sandstone facies rocks.
The distribution of the Upper Devonian deposits corresponds to that of the Middle
Devonian rocks, lacking only in areas where they have been destroyed by glacier. The
Upper Devonian is represented by the Frasnian and the Famennian stages. Frasnian
and Famennian deposits are rather various by origin and composition. At the very
beginning of the Frasnian a new record of mixed carbonate and siliciclastic sedimentation
started. Diversity of deposits is particularly characteristic for the Famennian, which is
composed of sandstones, dolostones, dolomite marls, siltstones, clays, limestones and
other rocks intercalating in a complicated pattern (Savvaitova 1977). The Famennian
basin was situated in the Latvian-Lithuanian Depression, which in the west and southwest
was periodically connected to the open sea. Hence, Famennian deposits are spread
only in south-western Latvia and north-western Lithuania. During maximum
transgressions the Baltic paleobasin was probably connected also with the sea of Middle
Russia through the Pripyat’ Deep (Savvaitova 1977).
The Ketleri Formation comprises sands, sandstones, clays, and dolomitic marls
which overlie the carbonate deposits of the Žagare Formation. In Latvia the thickness
of the Ketleri Formation reaches 45 m, while in Lithuania it is 41 to 56 meters thick.
The formation is divided into three members separated from each other by the erosional
surfaces (Lyarskaya and Savvaitova 1974). The lowermost, Nīgrande Member is
characterized by 8 to 12 meters thick clayey carbonate deposits which overlie an eroded
surface of carbonate rocks of the Žagare Formation. The lower part of the member is
composed of greenish grey and brownish dolomite marls with a low content of silt.
Among them several thin beds of greenish grey microcrystalline dolomite and clay
occur.
Tiny poorly preserved fish remains have been found in the dolomite marls at the
base of the member. P. Liepinš, E. Mark-Kurik and V. Talimaa have determined them as
belonging to the taxa: Chelyophorus sp., Bothriolepis ornata Eichwald, Devononchus
sp., Holoptychius cf. nobilissimus Agassiz, Eusthenopteron sp., Osteolepididae gen.
indet., Dipnoi indet., Palaeoniscidae gen. indet. (Liepiņš 1959; Savvaitova and Žeiba
1981). However, the identification of a species may be inaccurate and causes some
doubt. For example, the two species Bothriolepis ornata un B. ciecere Lyarskaja resemble
each other in respect to the structure of the head shield and ornamentation, but they
differ remarkably in the proportions of the trunk armour plates and character of dorsolateral
and ventro-lateral ridges (Lukševičs 2001). Quite often it is impossible to
determine the species by small fragmentary remains. L. Lyarskaya mentions
Devononchus tenuispinus (Gross) among the fish remains from the Nigrande Member
(Lyarskaya and Lukševičs 1992).
The amount of carbonate in rocks decreases towards the upper part of the member,
and dolomite marls are replaced by clay and clayey siltstone, in some cases by finegrained
sandy material. Spores of Stenozonotriletes sp. sp., S. triangularis Naum.,
S. simplex Naum., Euryzonotriletes sp., Trematozonotriletes intermedium (Waltz)