Origin
of the Deposit
Experts
have, over the past century, put forward conflicting opinions
as to the origin of this unique deposit. An early theory was
that of a mud volcano liberating an enormous volume of petroleum
and mineral matter in a state of protracted agitation during
which the volatile constituents were dissipated and the heavy
bituminous residue became uniformly homo-genized with the fine
clay and silica minerals, water and gas has now been superseded
by the conclusions of Dr. Kugler and others as a result of later
researches and more advanced environmental knowledge. Briefly,
this postulates the following theory. During Laramian folding
a gentle anticline of Upper Cretaceous and Paleocene rocks was
formed below the present Lake. This led to the separation of
gas, oil and water inside the structure. The Upper Eocene transgression
removed the Navet and part of the Chaudiere formations which
resulted in a dispersal of most of the gas and lighter oils.
Renewed folding
movements and additional fracturing took place prior to the
transgression of the Nariva sea when asphaltic viscous oil,
heavier than water: started to flow onto the shallow muddy sea
floor.
Sediments with their foraminiferal fauna covered repeated flows
of the oil kept in agitation by a continuous flow of gas, thus
mechanically mixing, detrital matter with the inspissated oil
and water to an emulsion. Due to its viscosity, the mass of
oil held together and it gradually grew to an inverted cone
surrounded by the increasing thickness of the sediments.
After the deposition of about 2,000 feet of sediments, it may
be assumed that the mass of asphalt was probably buried and
little seepage of oil took place. During the later Oligocene
structural movements, a further dispersal of Cretaceous oil
took place. Every crack and fissure was injected by plastic
bitumen that subsequently lost its volatile parts and formed
the
pencillate bitumen (Manjak) dikes partly found on the Upper
Cretaceous itself.
During
the latest Pliocene movements, it appears that the mass of asphalt
at the base of the Nariva beds had an opportunity to ascend
along the intersection of two major faults that met near the
centre of the present Lake. The asphalt flowed into the shallow
mangrove lagoon of Upper La Brea in time and grew again as an
inverted cone during the deposition of the youngest sediments.
It is also possible that the ascending asphalt mass simply lifted
the roof of the uppermost 300 feet of sediments which was carried
away by erosion.
Finally,
the Gulf of Paria broke in through the Northern Bocas (the narrow
sea passages separating Trinidad from the adjacent islands and
the mainland of Vene- zuela) and the erosive waves reached the
Pitch Lake area with its large surface sheet of asphalt covering
perhaps 3,000 acres. The asphalt flowed down the forming cliffs
and protected them, but large masses now lie in the sea off
La Brea village. The erosion swept east and west inland and
the peninsula of La Brea was formed and still protected by the
apron of asphalt. Today the Lake is situated about 1,200 yards
from the sea and occupies an area of about 90 acres in a depression
immediately to the south of a 140 foot high hill, from the summit
of which the ground slopes gently northwards to the sea. In
appearance the surface of the Lake is a uniform expanse of asphalt
which is intersected by areas of water, the extent of which
naturally varies according to the season.
Pumps
keep the Lake sufficiently free from water to enable digging
of the asphalt to be carried on, but at no time does the surface
become entirely dry and there is always water lying in the folds
of the asphalt. The major part of the Lake is hard enough to
walk on, but there are some small areas of soft materials which
are so soft as to be dangerous. Although quiescent the asphalt
still moves with a natural slow “stirring” action. Not only
can the “flow lines” be seen on the surface of the asphalt,
but prehistorical trees, etc. have been known, in the past
to have appeared, disappeared
and reappeared.
The
depth of the Lake was a matter of speculation in former times
and various attempts were made to reach the bottom of the deposit.
Through the use of modern technology and equipment, the depth
of the Lake is now known to be
The
asphalt itself is an emulsion of water, gas, bitumen and mineral
matter, the last name consisting largely of fine silica sand
and a lesser amount of impalpably fine clay. In some parts of
the Lake there is a small influx of soft material. This is accompanied
by a stronger evolution of gas consisting principally of methane
with a considerable proportion of carbon dioxide, and which
also contains hydrogen sulphide. This influx gradually hardens
and becomes like the main deposit of the asphalt. As this occurs,
fresh material breaks out elsewhere
END
