[Nmcaver] Guatemal City "Sinkhole"

[Minton, Mark]

      Below is a post from TAGNet with a likely explanation for the recent Guatemala City "sinkhole".

Mark Minton


3)The Guatemala City
      By: Ric Finch  (Cookeville, Tennessee)
          rfinch at tntech.edu

TAG Cavers-- The Guatemala City "sinkhole" is an impressive vertical
shaft, no doubt about that. But it probably should not be called a
sinkhole, and it probably is not a karst feature at all.

Guatemala City lies in an elongate, N-S trending, graben valley that
is partially filled with young (Quaternary age) volcanic deposits,
mainly pumice and ash. These deposits once formed a flat plain, but
surface streams have eroded them so that there are numerous deep and
steep-walled barrancas that dissect the ash plain into flat-topped
ridges. The so-called "sinkhole" formed in the barrio (neighborhood)
of San Antonio, which sits on one of these fairly narrow flat-topped
ridges. San Antonio barrio lies just west (500 m) of the Rio Las Vacas
and immediately north of highway CA-9 near where it exits Guatemala
City headed for Puerto Barrios on the Caribbean coast. This highway
crosses the Rio Las Vacas on a high bridge, Puente Belice, from which
you can probably see the San Antonio neighborhood if you look
northwest (downstream). The barranca here is about 80 m deep.  Note
that in the news the "sinkhole" is reported to be 330 feet deep, which
is fairly obviously a rounded conversion from 100 m. This even 100 m
is almost certainly a "guesstimate" and though the damn thing is
impressively deep, it may well turn out to be more like 80 m or some
depth related to the level of the Rio Las Vacas in this area. (My
reasoning behind this prediction will be made clear below.)

I have looked at a lot of photos of the Guatemala City "sinkhole" and
the best ones are those taken from a helicopter that allow you to look
down into the dramatic shaft. The walls are clearly volcanic ash
and/or pumice beds. Although some cavers discussing the news photos on
a caver forum have opined that the white material below what looks
like a thick red-brown soil profile must be limestone, it is not. The
ash and pumice deposits lying in the Guatemala City graben valley are
typically white colored, and sometimes brown or reddish brown. The
apparent thick soil profile is probably mostly ash too. These deposits
are very weak and crumbly, easily eroded, yet, like loess, they have a
degree of cohesion that allows them to stand in very steep to vertical
cliffs (seen everywhere in road cuts in the Guatemala City area), and
also allows subterranean chambers to be opened up in them (certain ash
deposits are mined, and the entrances to the hand-dug mines are a
common sight in road cuts, too).

The best photos I have found can be seen at this URL:
http://www.ordena.com/digg/sinkhole.html

The area of the San Antonio barrio is found on the San Jose Pinula
1:50,000 topographic and geologic quads. The GQ shows only Quaternary
pumice and ash cropping out at San Antonio. Underlying the easily
eroded ash deposits in many places throughout Guatemala City are older
(Tertiary), more solid and more impermeable lava flow rocks; I suspect
that these lava flows underlie the San Antonio barrio, but this cannot
be proven from merely observing the geologic map...it's just a logical
suspicion. Also in this general map area are very limited outcrops of
a Cretaceous limestone that was faulted and eroded long before the
volcanic layers were deposited. An outcrop at least a square kilometer
in area lies just a kilometer northeast of San Antonio, surrounded by
younger volcanic deposits, through which it peeks out like an
island...indicative of a surface of considerable relief on top of the
eroded limestone and below the volcanic deposits-- an erosional
unconformity, to use the proper geologic term. This limestone block
lies on the east side of the Rio Las Vacas canyon, whereas the San
Antonio neighborhood lies on the west side. Furthermore, a suspected
fault has been mapped along the Rio Las Vacas streambed, which, if the
geologist who mapped it was correct, also separates the limestone from
the location of the collapse shaft.

With all that as background, here then, is my seat-of-the-pants, viewed
from afar, interpretation of the Guatemala City "sinkhole":  A major
storm water and sewer main ruptured, releasing fluid into the porous
and easily eroded ash/pumice deposits underlying the flat ridge on
which San Antonio lies. This downward moving water eroded a shaft
below the ruptured drain. Shaft development likely occurred slowly at
first, but accelerated once some through-going system was established.
The water and the eroded sediments had to go somewhere, and that
movement was likely towards the Rio Las Vacas, lying approx. 80 m
below the general surface level of San Antonio, and only 500 m distant
laterally. The water moving downward through the ash/pumice deposits
may have reached the contact between the porous younger ash and the
older, less permeable flow rocks supposed to underlie the ash, and
followed this contact laterally to reach the Rio Las Vacas (which
would serve as the local base level). Once a good conduit was
established, one through which sediments could be moved, the
development of the shaft may have been rapid. Residents of the San
Antonio neighborhood report that they could hear noises underground
for a month prior to the final breakthrough to the surface, which was
indeed rapid and dramatic and took three unsuspecting residents down
to a horrid death.

The field datum I would like to have to confirm this interpretation
would be the discovery of a resurgence (especially a new and very
filthy stinky resurgence) somewhere along the west wall of the
barranca of the Rio Las Vacas.

Now, back to the limestone that crops out just a kilometer away. As
noted above, it is on the opposite side of the river and probably
separated from San Antonio and the shaft by a fault. Nonetheless, its
presence this close means we cannot rule out more of the same
limestone underlying the volcanic deposits on which San Antonio sits.
If limestone is under San Antonio, it does not crop out, and therefore
almost certainly must lie at some depth over 80 m. Still, it could be
there, and some pre-existing karst conduit could have been utilized by
the downward moving effluent from the burst pipe as its means of
escape. The water and the sediment had to go somewhere. Again, the
needed field datum is where is the resurgence? If it is related to
some karst feature in the Cretaceous limestone, the resurgence is
likely to be much further away, to the north, downstream from the more
proximate area where it might occur if my idea that the fluid followed
the contact between the ash and the flow rocks should be correct.

No doubt there will be geologic field investigations that will answer
some of these questions during the next few months or year or so.

Ric Finch
NSS 5560 RL and
Retired geology prof with a fair amount of experience in Guatemala