FOREWORD
Last
year, we were invited to propose a field trip for the International
Subcommission on Cambrian Stratigraphy (IUGS - UNESCO). We were aware of the little knowledge that some of our outcrops had
(compared to other parts of the world), but we also knew the very interesting
things that we have to show in the Andes of Argentina. Fortunately, our
proposal was accepted on the meeting held in Utah ("Laurentia 99"),
so with this field guide we welcome all participants of the field meeting and
hope you enjoy your stay with us. In this way, The Field Meeting Argentina
2000 of the ISCS, was programmed to be held in August 18 to 24, so, this is
the first time Cambrian specialists meet in an international gathering in
South America, and in a country exhibiting either remnats of the Laurentia
(Cambrian of Precordillera) as well as the Gondwana (Cambrian of the Eastem
Cordillera). Despite a lower attendance and a lesser representation from different countries of the
world, there are nevertheless a most varied technical program, with much new data presented, which is reflected in the
range of contributions submitted.
In a general sense, two Cambrian basins should be
recognized within Argentina: a carbonate dominated one, and a typical clastic
"Gondwanan" type:
- The first one is characterized by the highly
fossiliferous limestones and dolostones of the Precordillera of western
Argentina. It can be subdivided finto two different regions, one with typical
shelf facies (eastern and central Precordillera of La Rioja and San Juan Provinces), where the
stratigraphic sequence comprises, from the base upwards,
La Laja Formation (Lower to Middle Cambrian), Zonda Formation (Middle
Cambrian), La Flecha Formation (Upper Cambrian), and La Silla Formation
(Uppermost Cambrian to Tremadoc), which is succeeded by the limestone of the
Early Ordovician San Juan Formation, which include interbedded K-bentonite
levels. The other regions are located at the Western Precordillera of San Juan
and Precordillera of province of Mendoza, where the Cambrian deposits involves
basin and slope facies. The significant affinities between the Cambrian faunas
of the Precordillera with that of the Appalachian region from Laurentia, and
the K-bentonite beds appearing in the upper part of the San Juan Formation and
in the Gualcamayo Formation, allowed a strong relationship between them, to
consider the Precordillera as a continental fragment rifted from Laurentia.
- The second basan, in north-west and eastern
Argentina is not highly fossiliferous, and was developed in the Puna, Eastem
Ranges, Subandean Ranges, Famatina and the somehow disconnected Southern
Buenos Aires Ranges. In Northwest Argentina, they are characterized by over
1000 m of siliciclastic rocks that start with low grade metamorphic rocks with
trace fossils (Puncoviscana Formation), being followed by almost
unfossiliferous sandstones and quartzites (Mesón Group) and finishing with
the fossiliferous sandstones and shales that bear the transition to the
Ordovician System (lower part of Santa Victoria Group).
As an addition to the latter, intracratonic basins,
open shelves, continental active margine and the cratonized areas are the framework where sediments were deposited during the
Cambrian. Finally, deformation and metamorphic events, as well as a large
amount of igneous processes are also recorded during the Cambrian. The purpose
of the field trips is to give to all the participants a general overview of
the Cambrian rocks from Argentina, and to generate field discussions on
different aspects of our Cambrian units. A better general understanding of the
sedimentary sequences is needed, to interpret the particular mosaic that we
have in the Andean margin of South America.
The
Cambrian Period is one of the more significant and conspicuous in the Precordillera of Western Argentina and has many distinctive
charácteristics,
such as its geotectonic, paleogeographic and biostratigraphic features. The
Cambrian sedimentary sequence of the Precordillera includes, up to.date, the
only carbonate deposits known from South America, within the so-called
Gondwana continent. In this way, this sequence represent a widespread
development of carbonate platform on a Grenvillian basement, known from
Laurentia continent. In order to review current research and develop
strategies for future studies and international collaboration, the Field
Meeting Argentina 2000 "Precordillera and Eastern Cordillera of Western
Argentina" of the International Subcommission on Cambrian Stratigraphy
(ISCS), has been scheduled in the provinces of San Juan, Salta and Jujuy
respectively, where the members of the Meeting will have the opportunity to
discuss in the field, the different aspects related to Cambrian Period. The
present volume contains selected abstracts from the Meeting, and represent a
prelude of the main volume which will include the full papers.
The
precise recognition and definition of time in Stratigraphy is a continuing
priority in geological research. This has been accentuated in recent years by
the focus towards short-term geological and biological events in the
stratigraphic record. An important thrust of the Meeting is therefore on chronostratigraphy, with particular reference to the definition of the
Cambrian boundaries. In this volume, authors from Australia, Germany, Spain,
Denmark, United States, Sweden, korea, China, Argentina, Brazil, among
others, submitted papers contributing to the Cambrian Period knowledge. In
this sense, part of this volume is devoted to recent advances in Cambrian
paleontology and biostratigraphy. Emphasis is given on trilobite and conodont
faunas, but ichnological, micropaleontological, structural and isotopic
geochemistry studies are also included. The development of refined
geochronological methods is particularly noteworthy and illustrates the
potential for achieving and integrated, high precision chronostratigraphy and
geochronology for the Cambrian in the near future. This method could be a
valuable help in the precise correlation between basement rocks and
Precordillera, providing significant data for the paleogeographic
reconstruction
involving Gondwana and Laurentia.
These
field trips and volume could not have been brought to fruition without the
combined
efforts of the following people: First, the authors and field trips leaders
are thanked for their significant contributions leading the trips. We
gratefully acknowledge the sponsors for the Field Meeting Argentina 2000 by
the International Subcommission on Cambrian Stratigraphy (ISCS): The Secretaria
para la Tecnología, la Ciencia y la Investigación Productiva (SETCIP), The
Facultad de Ciencias Exactas, Físicas y Naturales of the Universidad Nacional
de San Juan, the Universidad Nacional de Tucumán, the Asociación Paleontológica
Argentina, the Asociación Geológica Argentina, the Instituto Superior de
Correlación Geológica - CONICET -, the Cooperadora of the Facultad de
Ciencias Naturales of the Universidad Nacional de Tucumán, as well as
technical assistance of Alejandro J. Toselli, Fernando Hongn, Vicente Mulet
and Daniel Ruiz Holgado, for their valuable help in different ítems related
to the Meeting. We also thank our Spanish colleagues Juan Carlos Gutiérrez-Marco,
Miguel Angel San José, Eladio Liñán and Rodolfo Gozalo for all their asístante
on the organization. Finally, we hope you enjoy this volume, and the Field Trips
scheduled
for the Meeting.
Silvio
H Peralta
Guillermo F. AceñolaZa
August 18, 2000.
