Download INTRODUCCIÓN: LOS ANDES

Some Geoscientific Insights
of the New Airport Area
By
Prof. Dr. Theofilos Toulkeridis
Director of the Center of Geodynamics, Volcanology and Geothermal Research /
USFQ
Co-Director of the Center of Geoinformation / USFQ
and GEOINFO
in collaboration with
M.Sc. Guillermo Rosero
Pacific Foundation (FUPAREMMI)
Ing. Giovanni Rosania
Pacific Foundation (FUPAREMMI)
Mag. Richard Resl
Director of the Center of Geoinformation / USFQ
and GEOINFO
Some Geoscientific insights of the New Airport Area
By
Prof. Dr. Theofilos Toulkeridis
1.0 GEOLOGICAL AND GEODYNAMIC SETTING OF THE NORTHERN
ANDES
The high peaks and volcanoes of the Northern Andes and the great earthquakes
along the Ecuatorian continental margin are dramatic manifestations of the oceancontinent convergence with speed rates of 50-70 mm / year of the oceanic Nazca plate
which subducts below the South American continent which simultaneously generates a
mainly E-W compression in the Northern Andean area (Fig. 1).
Fig. 1: The North-Central Andes.
The Andean Cordillera was formed in Ecuador following several pulses since at
least Cretaceous times (Western Cordillera), and probably Paleozoic time (Cordillera
Real). The mountainous Andean belt, the so-called Sierra, represents the core of the
Ecuadorian Andes, which is characterized by two parallel mountain ranges (the Western
Cordillera and the Cordillera Real) separated by a narrow valley (the Interandean
Depression) which wanes southwards.
Fig. 2: Distribution of oceanic and continental plates responsible for the geodynamic setting of Ecuador
and for the Interandean Depression in particular.
The Interandean Depression is an important extensional structure bounded by
active faults scarps and partly filled by continental volcanic and volcano-sedimentary
deposits that locally reach some thousands of meters in thickness (Baldock, 1983) and
are thought to closely reflect the younger tectonic history of the Neogene Arc of the
Ecuadorian Andes. The formation of the Interandean Depression dates back to the late
Miocene time (Baldock, 1985), when the whole area was affected by a MiocenePliocene compressional tectonic phase and later by a Pliocene-Pleistocene
uplift
(Cambell, 1974; Faucher & Savoyat, 1973). Furthemore, a number of transverse
discontinuities offsetted the northern Andes such as the dextral transcurrent NE-SW
Dolores Guyaquil Mega fault (DGM), represents the southern termination of the
Quaternary volcanism and of the Interandean Depression and the NW-SE Chota-Mira
tectonic line offsetting the northern portion of the volcanic front. Nonetheless, the
Ecuadorian main mountain chain forms an elbow located between the SSE-NNWoriented Peruvian Cordillera and the SSW-NNE direction of the Colombian Cordillera,
of which change is explained with a higher stress and deformation fields responsible for
the much higher seismic and volcanic activity of Ecuador relative to Peru and Colombia
(Fig. 2 & 3 & 4 & 5 & 6).
Fig.3: More detailed insight of the geodynamic setting of Ecuador.
Fig.4: Paleogeographic reconstruction demonstrating compressional regimes in the Ecuadorian mainland
due to the collision of the oceanic Nazca plate with the continent.
Fig. 5: Speed rates and directions of movement of the oceanic Nazca (and Cocos) plate.
Fig. 6: The Dolores Guayaquil Mega Fault (DGM) separates different parts of the Ecuadorian mainland in
a transformal way.
As the Ecuadorian volcanic belt is a result of the subducted and in the upper
mantle remelted oceanic Nazca plate, extends N-S along the whole country, with an
average width of about 80 km. The Quaternary activity is characterized by the building
of a great number of huge stratovolcanoes form the highest peaks in the northern Andes,
while they do not occur south of 2°S. The distribution of the volcanoes of Ecuador is
restricted to four distinctive zones of which each represents an own chain of volcanoes
that more or less in form of cordilleras. These consist of the so-called western volcanic
cordillera which includes e.g. the volcanoes Rucu and Guagua Pichincha, followed by
the volcanoes of the inter Andean valley which includes e.g. Chimborazo and Cotopaxi,
continuing with the eastern volcanic cordillera made up e.g. of Cayambe and Antisana
and ending finally with four amazonian volcanoes including the most active Ecuatorian
volcano Sanga
(Fig. 7).
Fig. 7: The Ecuadorian volcanic belt.
The new airport area has a direct or indirect influence of up to 15 different
volcanoes such as Guagua Pichincha, Cotopaxi, Cuicocha, Pululagua besides others.
The volcanoclastic products of the different types of volcanoes (stratovolcanoes,
calderas, volcanic domes etc.) cover the basal volcanic complex consisting of thick lava
pile with minor tuff intercalations and minor ignimbritic cover. A previously mentioned
thickness of at least 1000 m has been observed along the fault scarps of the Interandean
Depression, while the source areas from which these products were erupted are in detail
unknown
present. This roughly tabular volcanic sequence is furthermore faulted
and tilted.
2.0 GEOMORPHOLOGICAL AREA OF THE NEW AIRPORT
The area of the new airport is in the depressional zone of the eastern side of the
city of Quito inside the Interandean Depression in the so-called basin of
Guayallabamba. The area is bordered by the mountain of Tiopullo to the south,
mountain of Mojanda to the North
the two extremes the
valleys of Machachi, Los Chillos, Tumbaco - Pifo and Guayllabamba occur. The
hydrographic system is dominated by the river Guayllabamba which itself is formed
the union of the rivers San Pedro and Chiche, simultaneously defining the
northwestern end of the airport construction area. The rivers Guambí and Uravía,
origin derived from the western side of Puntas, represent the southwestern
and northeast-northwestern
of the construction area. They end up entering into
the Guayllabamba river to the northwest where they also form a small valley with
highly inclined and deep slopes as a result of a very strong fluvial erosion
as
tributary to the river Uravía where volcanic deposits are accumulated (Fig. 8 and 11).
3.0 GEOLOGICAL SITUATION OF THE PROJECT AREA
3.1 STRATIGRAPHY
Previous paleoecological and paleoclimatic studies of the Ecuatorian Andes of
the Late Pleistiocene time,
by Claudio Ochsenius, included also some
radiocarbon age dating of the contact zone of the Cangagua formation and the
sedimentary rocks of the river Chiche yielding an age of around 48.000 years (Bristow
Fig. 8: Stratigraphic profile #1.
Fig. 9: Part of the profile #1 showing the prominent andesite (flow) layers.
& Hoffstetter). This demonstates that as results of volcanism denominaded cangagua,
aeolic sediments were deposited dur
s of time due
to the reactivation of the various volcanoes of the
surrounding
s that these type
of sedimentation events are very common up to present time.
Fig. 10: Part of profile #1 which demonstates in a spectacular way the erosive occurring landslide-fatality
in which the main highway to the valley dissapeared in seconds
back
July 1999.
In order to demonstrate the potential
and partly given extreme instability of
the airport area two close to the airport occurring geologic-stratigraphic profiles were
chosen to become analyzed which will allow further insights about the subsurface
geological situation of the project area. The two chosen profiles appear at the road Quito
– Cumbayá – Tumbaco – Yaruquí, were analysed also for further discussion and
verification to the previous studies made by Ochsenius.
t 2.6 km distance of the partidero of Tumbaco at the 6th
ecember
avenue cropps out a lightgrey pyroxen bearing andesite which demonstrates a
with
s
rounded
surfaces
and
post-tectonic
transversal
. Above of these andesites follows an up to 20 m
thick layer of fluvial sediments with rounded fragments, evident for a long transport
way. Above these sequences occurrs an aeolic tephra layer denominaded cangagua with
a thickness of around 80 m. Below the road level, it is possible to observe a prominent
sequence of cangagua, lavas, tillites and other glacial deposits which extend more than
300 m.(Fig. 8-10).
For correlation purposes to this profile, we realised a further observation at the
bridge of the river Chiche at a distance of 18.1 km of the above mentioned partidero,
where the road cut allows to see a 80 m thick sequence of volcanic layers made up of
Fig. 11: Stratigraphic profile #2.
Fig. 12: Detailed view of the volcano-sedimentary / stratigraphic profile #2 at the Bridge of Chiche.
some fluvial sediments, aeolic volcanoclastic material and white lapilli. From the base
fluvial conglomerates are observable which are made of 5 to 20 cm big rounded
volcanic fragments.
this conglomerate, one finds a layer of aeolic
tephra, the so-called cangagua, of greyish color with an approximate thickness of 20 m.
Above this layer appears the deposition of a sequence of 2 m thick white lapilli,
overlayed by a thin horizon of a grey aeolic sediment and a thin layer of white lapilli;
above this layer appears again a layer of grayish aeolic sediment which itself
inclu
sediment.
other
greenish
Fig. 13: Further detailed view of the stratigraphic profile #2 at the Bridge of Chiche.
From
appears
an
aeolic tuffaceous sedimentary layer with an approximate thickness of 20 m, followed
from there by an aeolic sedimentary layer which is intercalated with pyroclastic material
and lavas. This sequence appears to have a thickness of around 100 m based on the
from the river level till the road (Fig. 11-13)
hanks to the pass and river cuttings
certain similarities
oth of these two profiles recorded
and they indicate that the past volcanic
activity has been severe resulting from different precursors and of different periodes of
time.
The profile of the river Chiche shows a sequence of basic (low SiO2) but
explosive volcanics, with interlayers of fluvial sediments and morrenes which of its
general character of being conglomerates are easily matters of erosive actions of their
matrix due to the presence of surficial and subterranean water. This allows clasts to
move freely away from its original strata which results to a flood producing
accumulation. Therefore this action leads naturally into landslides and to general
solifluxion phenomenas. This happened to be present in the first profile where morrenes
slided above competent layers of andesite, which therefore turned into a general
landslide which cutted of the road and of which some parts therefore dissapeared (e.g.
Fig. 10).
All units described so far, with exception of tha lavas, such as these in the above
mentioned paragraphs, are more likely semiconsolidated volcaniclastic accumulations
rather than real rock strata. All units with same rock and stability characteristics appear
below the platform area of the planned new airport. This leads to the conclusion, that
the base stability is potentially at least partly not given or very weak.
In order to determine the subsurface geology and therefore the erosional
record and also the stability of the Quito´s new airport area, a study of the
stratigraphic evolution of this continental volcano-sedimentary basin of probable
Miocene age is absolutely fundamental and necessary. Hereby a detailed
investigation of the depositional sequences such as the volcanic and volcaniclastic
layers, glacial and fluvial records in the subsurface area of the new airport need to
be exactly determined and evaluated as these are objects of prevention and
prediction of eventual risks and hazards which could affect future instalationes of
the surficial area.
3.2 TECTONICS
The construction site of the new airport of Quito is localised within the “ring of
Fire” of the Northern Andean Volcanic Zone at the NW end of the South American
continent. This area is the playground of the a highly active interaction of different
tectonic plates and terranes which are in permanent movement and drifting to and away
from each other with speed rates of 2 to 5 cm per year. One of the main consequences of
the collision of the plates involved is a general uplift of the whole area.
The interaction of the different plates have established a system of some main
(regional) geological faults of both types, strike-slipe (normal and reverse) as well as
transformal ones, followed by some minor faults of which last ones do appear in
perpendicular angle to the main faults, and which subsequently determine the stability
or unstability of the areas in which they occur (Fig. 14 & 15).
Fig. 14: Structural and tectonic zonation of Ecuador (and main volcanic distribution).
he
observation
pho
of
different
types
of
maps
and
air-
leads to the conclusion that
the causes of the rivers and gaps/valleys of the whole area which interfer in the one or
other form the platformal area originated predominantly by the presence of faults,
of which are even covered by landslides, micro-landslides and volcanic layers
as well as morrenes as result of the different glacial phases. In previous geological
maps not detected but obviously existent geological faults are evident and supported by
the abrubt changes of the flowing directions of the rivers San Pedro and Uravía, the
presence of inclined volcanic and morrenic layers and additionally the existence of a
series of tectonic terraces.
demonstrates clearly that the construction
area of the new airport is within a tectonically active zone
means that the
stablished of the constantly
active geological structures which occur in the project area, as well as their
distribution and territorial
of influence. A further observation is necessary
with the scope to find out about previous reactivations which certainly have occurred
and how these movement have affected human activities and constructed infrastructure
and in which way they might affect the area in the future if not correctional actions
taken into consideration from now on.
Fig. 15: “Three dimensional map” with main assumed and recognized geological faults without second
generation faults and microfaults.
During the study and interpretation of the sate lital images and subsequent
fieldwork the
regional structures and the circuit of action would be
matter of determination. Dealing with the geological faults it is absolutely obvious
and therefore necessarry to understand the potential and the extension of active
faults
and
their
secondary
generated
s a matter of
fact, this can be determined only with a detailedfieldwork, structural analyses,
some core drilling, geochemical sampling and all combined with GIS as proposed
in our submitted proposal.