J. Bek, M. Vavrdova, P.E. Isaacson, E. Diaz Martinez

Institute of Geology, Academy of Sciences CR, Rozvojova 135, 16502 Praha 6 <bek@gli.cas.cz>

Submitted Aug 15 / Accepted Sep 20, 2001

PALYNOLOGICALLY DOCUMENTED LATE DEVONIAN START OF LARGE GONDWANAN GLACIATION AND AGE OF UNDERLYING DEVONIAN FORMATIONS (LAKE TITICACA, PERU/BOLIVIA) 

The beginning of the main Late Palaeozoic glaciation in Gondwana presents a good stratigraphic record in the western sector of the supercontinent, which was at high latituted during the Devonian and part of the Carboniferous. Recent advances in the study of glacial marine deposits and their palaeoclimatic significance, together with the revised older ages of several South American stratigraphic units previously considered to be Pennsylvanian, provide further evidence for the Late Devonian glaciation in westernmost Gondwana (Peru and Bolivia).

The age of glaciation is constrained by invertebrate and palynomorph biostratigraphy. Late Palaeozoic glacial centers began to form during the Late Devonian in western Gondwana (northern South America and northern Africa), and shifted towards eastern Gondwana during the Carboniferous. 

Palynological data were utilized in the reconstruction of the sedimenatry palaeoenvironment in Bolivia during Devonian. Age significant species such Retispora lepidophyta have contributed to the recognition of correct chronological position of siliclastic units otherwise limited palaeontological record. 

The oldest sediments of the Tequeje Formation yielded assemblages of palynomorphs which indicate an open-marine, off-shelf environment. Palynomorphs of marine origin attain 95% of recovered palynomorphs (chitinozoans were abundant and well-preserved at this level). Most characteristic were, among other, Cingulochitina serata, Angochitina filosa and Urochitina loboi. These age significant species of miospores and of marine microplankton indicated Lochkovian and possibly latest Silurian age.

The upper part of the Tequeje Formation has been palynologically assessed with palynomorphs assemblages which exhibit a marine/terrestrial ratio of palynomorphs indicating a shallow, marginal marine facies. Miospores such as Geminospora lemurata and Acinosporites ledundae indicate a Middle Devonian age of the upper part of the Tequeje Formation.

The same sedimentary environment of circumpolar seas and shores persisted through deposition of the Tomachi Formation, but cooling is indicated with deposition of glacigenic material. Generally equal number of terrestrial spores and marine algal cysts have been recovered from this formation, together with abundant chitinozoans and terrestrial plant debris, such as cuticles and tracheids. Occasional blooms of prasinophyte algae (genera Maranhites, Tasmanites, Hemiruptia) mark the black shale horizons with abundant degraded organic matter. The Frasnian-Famennian Colpacucho Formation (in Lake Titicaca area) is an equivalent of the above Tomachi Fm. and contains isolated dropstones, and also aggreagted dumps from icebergs. The latest Famennian Cumana Fm. contains over 100 m thick series of marine glacial diamictites, and even the interleaving black shale layers contain numerous striated dropstones. The palynological materials from Colpacucho-Cumana, with spores and marine plankton specimens, are the main evidence for the early beginning of the Late Paleozoic glaciation. Start of this glaciation corresponds to Late Frasnian climatic oscillations and overall sea level drops which are largely observed on the globe.

The study contributed networking in the IGCP Project 421 "North Gondwanan Mid-Paleozoic Biodynamics Etc.".

LAKE TITICACA – SOUTH AMERICA / Location of investigated glacigenic sediments in the southwestern circumpolar part of the Late Devonian (Frasnian?, Famennian, LN palynozone mainly) to Early Carboniferous (Tournaisian) Gondwanan shores

Tumulispora rarituberculata Playford, 1991 / an example of miospore objects from the end-Devonian times / Jiri Bek

Convolutispora flexuosa Hacquebard, 1961 / an example of miospore objects from the end-Devonian times / Jiri Bek

Umbellasphaeridium saharicum Jardiné et al., 1972 / an example of microplankton objects from the end-Devonian times / Milada Vavrdova

Exochoderma irregulare Wicander, 1974 / an example of microplankton objects from the end-Devonian times / Milada Vavrdova