CHAPTER 2 • The Maracaibo System: A Physical Profile
17
and flow into the western side of the lake. A predominant feature of the south-western corner of the lake is the Catatumbo River, whose birdfoot delta is similar in shape
and sedimentology to large oceanic deltas, like the Mississippi's (Hyne and Dickey
1977). The only stream of importance on the western side of the lake is the Palmar
River. In the south-east the area between the lake and the bordering mountains is rather
narrow and streams are short and fast flowing (Gonzalez de Juana et al. 1980).
The lake bottom is pan-shaped, and nearly flat over two-thirds of its surface. The
flat bottom extends to the central and eastern portions and corresponds to water
depths of about 30 m. A narrow, relatively steep, shoreward rising of the lake bottom
characterizes the eastern and southern borders. In the north-western portion the
bottom forms a gentle and broad incline. A sill, about 12 m deep (Redfield et al. 1955),
separate the lake bottom from the Strait of Maracaibo.
Even though the lake bottom is nearly flat, in detail it has irregularities. These. bottom features consists of fairly large elongated depressions, smaller nearly circular holes,
and small mounds. Some of these depressions are 4 km in length with a maximum
relief of 10 m. This feature may be due to differential compaction of the Recent sediments. The series of small depressions or holes found in some areas may be due also to
compaction phenomena or related to springs or gas seeps (Sarmiento and Kirby 1962).
2.3
Geological History of the Estuary
The Maracaibo Basin occupies an old platform located perypherically to the Precambrian Guiana craton which was covered during most of the Cretaceous transgression by epicontinental seas. It reached a maximum submergence during the Cenomaniense-Santoniense (100 my) This platform has been sinking since the end of the
Eocene at a rate of 0.1 mm ye l (Sutton 1946), and is covered at present by a thickness
of sediments of approximately 5000 m (Zuloaga 1950).
The structural Maracaibo Basin was formed during Mio-Pliocene time, when the
Venezuelan Andes and the Sierra de Perija attained their maximum development. The
formation of the basin changed the sedimentary environments from restricted marine and transitional to predominantly transitional and continental. This type of sedimentation has continued into present times resulting in a gradual filling of the slowly
subsiding basin (Graf 1969). The Gulf of Venezuela has retained its present form from
Pliocene times.
At the beginning of the first eustatic regression of the Quaternary (Nebraska glaciation), the Calabozo Ridge, which partially separate Calabozo Bay from the outer
Gulf of Venezuela, was partially emerged, forming a coastal lagoon, or endorheic basin, completely separated from the open sea (Fig. 2.2). During the first interglacial
(Aftonian) the sea transgreded again and the interior basin (Lake Maracaibo) was
transformed into a coastal lagoon. This cycle repeated itself with the following eustatic
variations of sea level, up to the last postglacial transgression which began with the
Holocene, approximately 20000 years B.P. (Graf 1969,1972).
The Holocene sediments of Lake Maracaibo attain a thickness of as much as 20 m
and consists of four stratigraphic units: A clay substratum, probably of Pleistocene
origin and belonging to the upper part of the so-called Milagro Formation; a thin unit
consisting of peat, clay and sand; soft unconsolidated muds which made the bulk of
17
and flow into the western side of the lake. A predominant feature of the south-western corner of the lake is the Catatumbo River, whose birdfoot delta is similar in shape
and sedimentology to large oceanic deltas, like the Mississippi's (Hyne and Dickey
1977). The only stream of importance on the western side of the lake is the Palmar
River. In the south-east the area between the lake and the bordering mountains is rather
narrow and streams are short and fast flowing (Gonzalez de Juana et al. 1980).
The lake bottom is pan-shaped, and nearly flat over two-thirds of its surface. The
flat bottom extends to the central and eastern portions and corresponds to water
depths of about 30 m. A narrow, relatively steep, shoreward rising of the lake bottom
characterizes the eastern and southern borders. In the north-western portion the
bottom forms a gentle and broad incline. A sill, about 12 m deep (Redfield et al. 1955),
separate the lake bottom from the Strait of Maracaibo.
Even though the lake bottom is nearly flat, in detail it has irregularities. These. bottom features consists of fairly large elongated depressions, smaller nearly circular holes,
and small mounds. Some of these depressions are 4 km in length with a maximum
relief of 10 m. This feature may be due to differential compaction of the Recent sediments. The series of small depressions or holes found in some areas may be due also to
compaction phenomena or related to springs or gas seeps (Sarmiento and Kirby 1962).
2.3
Geological History of the Estuary
The Maracaibo Basin occupies an old platform located perypherically to the Precambrian Guiana craton which was covered during most of the Cretaceous transgression by epicontinental seas. It reached a maximum submergence during the Cenomaniense-Santoniense (100 my) This platform has been sinking since the end of the
Eocene at a rate of 0.1 mm ye l (Sutton 1946), and is covered at present by a thickness
of sediments of approximately 5000 m (Zuloaga 1950).
The structural Maracaibo Basin was formed during Mio-Pliocene time, when the
Venezuelan Andes and the Sierra de Perija attained their maximum development. The
formation of the basin changed the sedimentary environments from restricted marine and transitional to predominantly transitional and continental. This type of sedimentation has continued into present times resulting in a gradual filling of the slowly
subsiding basin (Graf 1969). The Gulf of Venezuela has retained its present form from
Pliocene times.
At the beginning of the first eustatic regression of the Quaternary (Nebraska glaciation), the Calabozo Ridge, which partially separate Calabozo Bay from the outer
Gulf of Venezuela, was partially emerged, forming a coastal lagoon, or endorheic basin, completely separated from the open sea (Fig. 2.2). During the first interglacial
(Aftonian) the sea transgreded again and the interior basin (Lake Maracaibo) was
transformed into a coastal lagoon. This cycle repeated itself with the following eustatic
variations of sea level, up to the last postglacial transgression which began with the
Holocene, approximately 20000 years B.P. (Graf 1969,1972).
The Holocene sediments of Lake Maracaibo attain a thickness of as much as 20 m
and consists of four stratigraphic units: A clay substratum, probably of Pleistocene
origin and belonging to the upper part of the so-called Milagro Formation; a thin unit
consisting of peat, clay and sand; soft unconsolidated muds which made the bulk of