Biology Reference
In-Depth Information
reconstruct the Quaternary paleoenvironments of central and northern
Mexico.
To the south of the Transvolcanic Belt is the Balsas Depression through
which the Balsas River fl ows. It has been dammed and is now a major
source of hydroelectric power for the region. Farther south are the hot,
windy lowlands of the Isthmus of Tehuantepec. The average elevation is
about 80 m, and it was one of four sites considered for construction of
the transoceanic canal. The others were in Nicaragua, the Darién region of
easternmost Panama and adjacent Colombia, and the eventual site across
central Panama. In a twist of fate with great political and economic conse-
quences, the United States Congress met on 9 May 1902 to choose between
the fi nal candidates of Nicaragua and Panama. The day before, on 8 May,
Mt. Pelée on the Lesser Antilles island of Martinique erupted, and with the
devastation so vividly evident, the more quiescent Panama was selected as
the safer route.
Just north of the Isthmus of Tehuantepec is the Sierra Madre del Sur.
These mountains probably originated as an offshore island arc and were
added onto the mainland in the Late Cretaceous. It is a rugged region, with
elevations to 3703 m north of Acapulco, and there have been numerous
earthquakes, including one of 7.8 magnitude on 29 November 1978. Just
south of the isthmus is the Sierra Madre de Chiapas. It originated by com-
pression from the south, primarily during the Miocene, to form a series
of plateaus, depressions, and steep-faced cuestas evident in the distinctive
up-and-down topography along the road from Tonalá to the Guatemalan
border. East of these mountains is the Central Depression through which
the Grijalva River fl ows and across which Mexico's largest dam has been
built. Most of the region was submerged in the Cretaceous and emerged
in the middle and late Tertiary. To the east is the Coastal Plain and the
Yucatán Platform. The position of the shoreline in Eocene and later times
is marked by a series of lignites containing
Rhizophora
and other vegeta-
tion typical of coastal environments. These sediments extend northwest
to southeast between the villages of Simojovel and Ixtapa and are a full
90 km inland from the present coast. The shoreline to the north in the
middle Eocene is identifi ed by lignites and associated sediments of the Lar-
edo Formation of Texas and the Burgos Basin of Nuevo Laredo, Mexico,
containing remains of sharks, skates, rays,
Tarpon
, Crocodylidae, and the
present-day Old World mangrove palm
Nypa
. Between Nuevo Laredo and
Simojovel, the retreating shoreline had reached to within 1-2 km of its pres-
ent position by the middle Pliocene (4-3 Ma). This is shown by
Rhizophora
-
containing lignites, alternating with near-coastal sediments containing ma-
