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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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