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Brian Holland, FGS, Technical Director
& Chief Geologist
Belize Minerals Ltd.
EXECUTIVE SUMMARY
I have reviewed the AMEC geology
reports and feasibility studies for
the Chalillo dam and its reservoir basin. My evaluation includes a
review of the relevant geological literature, an examination of the
drill cores taken at the proposed dam site and observations made during
two visits to Chalillo.
The AMEC geology reports and feasibility studies are gravely flawed and
so filled with errors so as to render it useless as a document for
engineers to use in the design and the construction of the proposed dam.
The mistakes made by AMEC in the geological mapping of the Chalillo site
would get a failing mark in an introductory geology class.
AMEC has incorrectly mapped and identified the geology at the dam site.
There is no granite at Chalillo as AMEC states. The bedrock geology is
composed of sandstones, conglomerates and soft shales. No thermal
alteration (contact metamorphism) has ever been recorded in these rocks.
At Chalillo these rocks vary (laterally and vertically) from hard,
well-cemented rocks to those which are so friable they can be crushed by
hand. The presence and distribution of these weaker rocks has been
completely ignored by AMEC. Consequently, the AMEC documents should not
be used as a basis for the design and construction of the dam.
AMEC refers to the two main geological studies on the area, viz., Dixon
(1955) and Bateson and Hall (1977) but do not explain why their
interpretation of the geology at Chalillo differs so significantly from
these authors. AMEC make no reference to any of the many other
geological reports, publications and theses that deal specifically with
this area.
AMEC does not provide a geological map of the Macal - Raspaculo River
reservoir basin. The basin is known to contain widespread and thick
sequences of karstified Cretaceous limestones that unconformably overlie
the [Santa Rosa Group] sedimentary rocks in the Macal and Raspaculo
River beds. Without a detailed geological map of the reservoir basin it
cannot be assumed that water will not flow out of the reservoir and into
the immense cavern systems known to exist in the limestone formation.
AMEC incorrectly states that there are no faults or fractures at the dam
site. There are abundant faults and fractures (joints) that affect the
bedrock at Chalillo, features published by Dixon (1955) and Bateson and
Hall, (1977) and described in many other reports and theses on the
geology of the area. As seen in the drill cores, some of the fractures
are open to a depth of at least 45 meters (deepest core drilling). The
fractured bedrock is weathered down to that depth.
AMEC makes no mention of the more than 30 kilometer long Cooma Cairn
Fault located less than 1 kilometer to the west of the proposed dam
site. This dip-slip fault, which may have a displacement of up to 6
kilometers, is large enough to be seen on satellite images of Belize.
Introduction
The following is an expanded version of a report submitted to the
Natural Resource Defense Council on December 9, 2001. This report
includes my review of the relevant geological literature of the area,
inspection of the drill cores taken at the proposed Chalillo dam site
(report to Belize Electricity Ltd. November 16, 2001) as well as
observations made during two visits to the Chalillo site (December 15,
2001 and January 15, 2002). At the request of BACONGO (Belize
Alliance of Conservation Non - Governmental Organizations) I
prepared (October 26, 2001) a technical review of the geology
sections of the Chalillo dam EIA. This report was presented by
BACONGO to NEAC (National Environmental Assessment
Committee).
Background
Natural Resource Defense Council has asked me, as a professional
geologist, to provide a technical comment on the geological studies
presented by AMEC to Belize Electricity Limited (BEL) as part of the
feasibility study for a dam on the Macal River. The proposed dam site,
located in western Belize, is known as Chalillo.
As part of the planning to build a dam on the Macal River in western
Belize, Canadian consultant companies AMEC E & C Services Limited and
AGRA were commissioned by Belize Electricity Limited (BEL) to undertake
feasibility studies. In 1999 and 2001 the companies submitted reports to
BEL that, inter alia, purport to describe the geology of the proposed
dam site at Chalillo as well as the upstream reservoir basin. Another
company, Swissboring Overseas Corp., carried out 7 core drillings and
made a geological description and map of the dam site, although the name
AGRA also appears on the Swissboring geological map. It is not clear (at
least to me) from the documents exactly what work these various
companies have done on the project.
These documents are:
1. Macal River Upstream Storage Facility – Feasibility Study – Final
Report – Volume 1 – Main Report, AMEC E&C Services Limited, Canadian
International Development Agency, August 2001.
2. Macal River Upstream Storage Facility Environmental Impact Assessment
– Part 2 Support Documents – Volume I of IV, Belize Electricity Company
Limited (BECOL), AMEC E&C Services Limited, Canadian International
Development Agency, Industrial Cooperation Division August 2001.
3. Macal River Upstream Storage Facility, Dam Site Geotechnical Program
Report, Final Report. Belize Electricity Limited, AGRA CI Power,
Swissboring Overseas Corporation. November 1999
Previous geological work in the area
The proposed dam site is located in western Belize in the southern part
of an area known as the Mountain Pine Ridge. Bedrock in the area is cut
by a major NE-SW trending fault, named the Cooma Cairn Fault by Dixon
(1955).The uplifted western part of the area is the Mountain Pine Ridge
granite batholith while the downfaulted area to the east and southeast
comprises a Carboniferous-Permian sedimentary rock sequence known as the
Santa Rosa Group (see attached cross-section and stratigraphic column
taken from Bateson and Hall, 1977).
Prior to the AMEC studies carried out for the dam, the area had been
geologically mapped and studied by numerous researchers (Dixon, 1955;
Bateson and Hall, 1971; Bateson and Hall, 1977; Kesler et al, 1971;
Shipley, 1978; Dawe, 1984 and Nelson, 1984). [NB: Shipley did his study
as part of a Master of Science degree at the Colorado School of Mines
while Dawe and Nelson did their work at the Department of Geology, State
University of New York at Binghamton, New York]. Holland (1993) briefly
described the geology of the Macal and Raspaculo Rivers from Rubber Camp
on the Macal River up to the vicinity of the headwaters of the Raspaculo
River. Donnelly and others (1990) provide a useful synopsis of the
geology of Belize and especially the Maya Mountains. Two mining
companies, Duncan Derry Ltd. (London Tin Corp.) and Anschutz Corp. have
also mapped the geology of the area in detail. All of these reports are
housed in the library of the Geology and Petroleum Department, Belmopan,
Belize.
Sedimentary rocks form the bedrock geology at Chalillo
The proposed dam site at Chalillo is situated in the Macal River valley
which extends along the southern flank of the Mountain Pine Ridge.
The geology of the site has been mapped by all of the abovementioned
authors as rocks belonging to the Santa Rosa Group - clastic
sedimentary rocks (sandstones, conglomerates and shales) with some
subordinate limestone units. The Santa Rosa Group is dated as Permian
on the basis of fossils (see Bateson & Hall, 1977 and references
therein). The sandstones are referred to (op cit) as greywackes; the
coarser conglomeratic units I interpret as resedimented conglomerates.
Thin sections of the greywackes are shown in Bateson and Hall (1977,
page 6 Plate 2).
The sedimentary rocks of the Santa Rosa Group in the area of the
proposed dam have been deposited in submarine fans (Nelson, 1984).
The coarser sediments, the greywackes and conglomerates are not
blanket-type deposits that are thick, widespread and uniform, but
instead have most likely been deposited in channels eroded in the
underlying soft black laminated shales. This is evidenced by the sharp,
erosive contacts between the shales and greywackes and conglomerates
seen in the cores (see e.g. Walker, 1975, 1979).
Bateson and Hall (1977, page 4-5) offer the following description of
these clastic sedimentary rocks in the Mountain Pine Ridge area:
“The conglomerates are associated with sandstones (Plate 21. And 2.2),
most of which, and particularly those of a medium and coarse grain size
(average grain size greater than 0.3 mm), show a poor degree of sorting
with the groundmass consisting of fine granular quartz usually with much
sericite and occasionally muscovite. The coarser grains in these rocks
are up to 1 mm in diameter but generally show only a minor degree of
rounding. Quartz grains are dominant, but in most of the rocks there are
also very variable amounts of feldspar and rock fragments.
These rocks represent sediments which accumulated rapidly with a minimum
time for sorting and for chemical disintegration. Most of the coarser
sandstones are subgreywackes, but sorting improves with reduction in
grain size and some of the fine, well-sorted sandstones would probably
be classified as orthoquartzites. These rocks are characteristic of the
subgreywackes (Pettijohn, 1957) in that they do not show very many
sedimentary structural features. Some small scale current bedding has
been observed as has the local development of ripple markings. Grading
of beds was also recorded in those areas where there are sufficiently
good exposures to enable the transition from sandstone to mudstone to be
observed. Similarly the coarser and usually less well-sorted of these
sediments may lie on the channeled surface of the finer beds beneath,
and in these conditions the base of the greywacke often shows fluting
and other types of sole markings.”
There is no granite at Chalillo. The Mountain Pine Ridge granite does
outcrop circa 1 kilometer west of the Chalillo site on the west side of
the Cooma Cairn Fault where Santa Rosa Group rocks are absent. The
Mountain Pine Ridge Granite has been dated by isotopes as early
Devonian, while the Santa Rosa group is of Permian age. Thus, there is
an age difference of nearly 140 million years between the older granite
and the younger Santa Rosa Group. There are [younger] Triassic granites
in the Maya Mountains that do intrude the Santa Rosa Group; these are
the Hummingbird and Cockscomb granites that occur miles to the east of
the Mountain Pine Ridge area (Dixon, 1955; Bateson and Hall, 1977).
Around these granites the Santa Rosa Group has been thermally altered or
contact metamorphosed, this type of alteration is not seen in the
sedimentary rocks at Chalillo
As mentioned above, Nelson (1984) interpreted the Santa Rosa Group
sedimentary rocks as having been deposited in a series of westward
prograding submarine fans. A submarine fan depositional environment was
also inferred for a similar Carboniferous –Permian sequence in an area
of the Maya Mountains a few miles to the south of the proposed dam site
(Miller et al, 1996).
Shipley (1978), in his study of the geology, petrology and geochemistry
of the Mountain Pine Ridge batholith, found no granite bedrock
outcropping at the proposed dam site. His map shows the dam site area as
“Santa Rosa Group, greywackes and quartzites”.
AMEC has mapped the site as granite
In contrast to all of the above studies, AMEC has mapped the Chalillo
dam site as uniform granite with xenoliths (inclusions) of sedimentary
rocks. Their study provides the following description of the geology at
the proposed dam site (page 78 in the August 2001 report), see also Site
Geology Plan Figure 2.2, by AGRA CI Power, Macal River Upstream Storage
Facility - Feasibility Study – Final Report.
“At the proposed dam site area, the outcrops forming the floor of the
river valley consist almost entirely of granitic intrusives. Below the
valley floor, some Santa Rosa sedimentary rock layers or inclusions were
found in the granite, but in relatively minor amounts. Above the valley
floor, however, Santa Rosa sedimentary rock constitutes a large
proportion of bedrock forming both rock abutments.
The Santa Rosa sedimentary strata consist of sandstone, shale,
siltstone, conglomerate and breccia having subhorizontal stratification.
Granitic intrusive layers also appear parallel to this stratification.
The stratigraphic position of the granite with respect to the Santa Rosa
sediment is uncertain. The granites from intrusive layers within the
sedimentary strata; they also contain what appear to be sedimentary
inclusions, which appear to underlie Santa Rosa sedimentary strata”.
If granites had indeed intruded these rocks one would expect to find
evidence of thermal alteration adjacent to the granite. Along with the
other authors mentioned above, I can categorically state that there is
no thermal alteration of any of the sedimentary rocks at Chalillo. This
is because no granite has intruded these rocks.
Core drillings at the dam site
The AMEC geological study of the proposed dam site appears to be based,
at least in part, on core descriptions provided by the core drilling
company Swissboring. Swissboring carried out 7 core drillings spread out
across several hundred meters of the Macal River at the proposed dam
site and up onto the river banks (drilling locations are shown on Site
Geology Plan Figure 2.2, by AGRA CI Power, Macal River Upstream Storage
Facility - Feasibility Study – Final Report).
The Swissboring report describes the predominant rock in Chalillo Core
Drilling no. 1 as:
“Granite, dark gray with white crystals, porphyritic, medium to coarse
grain, crystalline, fresh, strong to very strong, with few inclusions of
quartz and intrusive rock, up to 5 cm in size.”
The geologist further describes a layer in the granite at 10.50 meters
to 11.10 meters as:
“Shale, black to dark gray, fresh, moderately weak to moderately strong,
dip at 30 degrees, interbedded with granite (5 cm thick), fresh,
strong”.
In November 2001 I was asked by BEL to examine the same core drilling (Chalillo
Core Drilling no. 1). I found the entire core to be composed of
sedimentary rocks: greywackes and resedimented conglomerates and soft,
black, thinly laminated pyritic shales.
I also noted similar black shale sections recovered in cores in the
other drillings. For example, Chalillo Core Drilling no. 7 recovered a
black shale sequence more than 2 meters thick. No granite bedrock was
penetrated in any of the drillings. All of the rocks described as
granites by Swissboring are, in fact, sandstones (greywackes) and
resedimented conglomerates. In places the greywackes and conglomerates
are well cemented and hard; however, the degree of hardness or
cementation fades laterally and vertically at Chalillo so that the rocks
become friable and crumbly. This can be seen on the river bank by
Ballarina Road at the dam site, an area mapped as granite by AMEC.
Faults and joints
The AMEC reports state there are no faults or joints within the area
[they] mapped at the dam site. This is not true, there are numerous
faults at the dam site as well as abundant fractures (joints), both open
and quartz filled (a structural analysis of the features is provided by
e.g. Dawe, 1984). The prominent direction of the fractures is N-S. Dixon
(1955) and other authors map a fault that cuts (NW-SE) directly through
the Chalillo site.
Displacements of some of the fault are clearly shown by offsets of
quartz veins. The offsets range from a few centimeters to nearly 1
meter. One of these faults [with a meter sized displacement] can be seen
on the east side of Ballarina Road a few meters above the river bank at
the proposed dam site. The fault occurs in a friable greywacke with
abundant cobble sized clasts. Obviously, the displacement of the quartz
veins shows that the area has been tectonically affected after the
formation of the quartz veins.
A major fault, the Cooma Cairn Fault, is located approximately one
kilometer downstream of Chalillo. This fault was first mapped by Dixon
(1955). This NE-SW trending dip-slip fault is easily discernible on
satellite images of Belize. The Permian sedimentary Santa Rosa Group in
the vicinity of the Mountain Pine Ridge batholith is approximately 6
kilometers thick (Nelson, 1984); consequently the Cooma Cairn Fault may
have a displacement of at least this magnitude.
No thermal alteration of sedimentary rocks
AMEC claims the few sedimentary rocks they recorded are xenoliths in the
granite. If granite did indeed intrude these rocks, then one would
expect the black shales to be contact metamorphosed. This is not the
case; the shales are fresh, soft and show no signs of thermal
alteration.
In this respect, it is noteworthy that
Dawe (1984), in his study of the geology of the Mountain Pine Ridge
area, found no evidence of a contact aureole between the Mountain Pine
Ridge granite and Santa Rosa sediments nor chiastolite or andalusite in
stream sediment samples in the area (a feature that is common in Santa
Rosa Group rocks around younger granites elsewhere in the Maya
Mountains).
Implications of the geology for the design and construction of the
dam
Both outcrops and cores show that the bedrock at the proposed dam site
is not a uniform granite but is instead composed of layers of
sedimentary rocks (sandstones, conglomerates, and thinly laminated
shales) with greatly varying physical characteristics - the rocks range
from hard to very friable to soft (can be crumbled in the hand).
The soft and friable rocks have been largely ignored by AMEC as far as
strength testing is concerned. Clearly, the entire sequence of rocks
must be considered in a geo-technical evaluation of the site for a dam.
Furthermore, features such as the intense fracturing and faulting at the
site must also be included in an evaluation. Many of the fractures have
remained open deep below the surface and have allowed weathering of the
rock to take place at depth. In some instances the shales have been
altered to a clay consistency. The AMEC report takes none of these
important features into account or even offers a comment on their
existence.
Reservoir basin
AMEC has not provided a geological map of the dam reservoir basin along
the Macal and Raspaculo Rivers. The area has been geologically mapped by
Dixon (1955) and Bateson and Hall (1977). Large areas on both sides of
the Raspaculo River and south of the Macal River contain thick sequences
of karstified Cretaceous limestones unconformably overlying the Santa
Rosa Group sedimentary rocks. The karstified limestone in this region of
Belize contains the world’s 5th largest cave systems (see various
articles in Journal of Cave and Karst Research; special theme issue on
Belize 1966 Vol. 58, no.2). Clearly, there exists a potential for water
in the reservoir to leak away via these extensive karst systems.
Furthermore, the data gathered and presented by AMEC are insufficient to
determine or evaluate the possibility for the sedimentary rock sequence
at the dam site to include karstified Permian limestones deeper than 45
meters (deepest core drilling). These limestones were mapped and
described by Dixon (1955) and Bateson and Hall (1977) from a lower
stratigraphic section further upstream near Rubber Camp.
Conclusions
My conclusions after reviewing the AMEC geology report may be summarized
as follows:
The AMEC geology report and feasibility report are so filled with
fundamental errors and flaws so as to render them useless as a basis for
engineers to use in the design and the construction of the proposed
dam. The mistakes made in the mapping of the Chalillo site and in the
geological report would get a failing mark in an introductory geology
class.
AMEC has incorrectly mapped and identified the geology at the dam site.
There is no granite at Chalillo as AMEC states. The bedrock geology is
composed of channel fill and interbedded greywackes (sandstones),
resedimented conglomerates and soft shales deposited in a submarine fan.
No thermal alteration (contact metamorphism) is present in these rocks.
These rocks very in hardness vertically and laterally to include rocks
so friable they can be crumbled by hand. AMEC has not evaluated the
strength or distribution of these rocks at the dam site.
AMEC refers to Dixon (1955) and Bateson and Hall (1977) but makes no
reference to the fact that their interpretation of the geology at
Chalillo differs significantly from these authors. AMEC furthermore does
not refer to any of the many other geological reports, publications and
theses that deal specifically with this area.
AMEC does not provide a geological map of the reservoir basin. The basin
is known to contain large areas of karstified Cretaceous limestones
unconformably overlying the Santa Rosa Group rocks in the Macal and
Raspaculo River beds. The karst systems that permeate these rocks could
potentially allow reservoir water to leak away.
AMEC incorrectly states that there are no faults or fractures at the dam
site. There are numerous faults and fractures (joints) in the bedrock at
Chalillo. Some of the fractures are open to a depth of at least 45
meters (deepest core drilling). All 7 core drillings penetrated
fractured bedrock.
AMEC makes no mention of the Cooma Cairn Fault located less than 1
kilometer to the west of the proposed dam site. This dip-slip fault,
which may have a displacement of up to 6 kilometers, is large enough to
be seen on satellite images of Belize.
REFERENCES CITED
Bateson, J. H. & Hall, I. H. S., 1971. Revised geologic nomenclature
for Pre-Cretaceous rocks of British Honduras. Bull. Am. Assoc. Pet.
Geol. Vol. 55, pp. 529-530.
Bateson, J. H. & Hall, I. H. S., 1976. The geology of the Maya
Mountains, Belize. Overseas Memoir Inst. Geol. Sci. No. 3.
Donnelly, T. W. Horne, G. S., Finch, R. C. and Lopez-Ramos, E,
1990. Northern Central America; the Maya and Chortis blocks, in
Dengo, G. and Case, J. E., eds. The Caribbean Region: Boulder,
Colorado, Geological Society of America, The Geology of North
America, Vol. H.
Dixon, C. G., 1955. Geology of southern British Honduras with notes
on adjacent areas. Belize: Government Printer.
Holland, B., 1993. Geological observations made along the Rspaculo
River, May 1993. In: Joint Services Expedition to the upper Raspaculo
River. Natural History Museum, London.
Kesler, S.E., Bateson, J.H., Josey, W. L., Cramer, G. H. and
Simmons, W. A., 1971. Mesoscopic structural homogeniety of Maya
Series and Macal series, Mountain Pine Ridge, British Honduras. Bull.
Am. Assoc. Pet. Geol., Vol. 55, No. 1, pp 97-103
Miller, C. G, Fortey, P. L., Whittaker, J. E., Williams, J. E. and
Herrington, R., 1996. Sedimentological and palaeontological report on
samples from the Santa Rosa Group of the Maya Mountains, Central
Belize. Report to BHP Minerals International.
Nelson, J. R., 1984, Sedimentology and stratigraphy of the late
Paleozoic rocks of the Mountain Pine Ridge, Belize (Masters Thesis)
Binghamton, State University of New York at Binghamton, 52 pages.
Walker, R. G. 1975, Conglomerate: sedimentary structures and facies
models. In Depositional environments as interpreted from primary
sedimentary structures and stratification sequences. J. C. Harms, J. B.
Southard, D.R. Spearing and R.G. Walker, pp. 133-61, Short Course notes
no. 2 Tulsa: Soc. Econ. Paleont. Minerals.
Walker, R. G. (ed) 1979. Facies Models. Waterloo, Ont. Geol.
Assoc. Canada.
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