Geology and soil parent materials

Solid Geology

There are only a few outcrops on BCI, nearly all in streambeds Most of the following geological summary is based on Woodring’s (1958) geological map of BCI. We have made a few adjustments based on streambed outcrops and surface stones, as seen in 2005-6, mainly with respect to the andesite-Bohio boundary in the area NE of the radio mast.

BCI lies at the eastern end of the Chorotega Block, part of a volcanic arc that connects North and South America (Coates & Obando, 1996). The arc results from the collision of the South American, Nazca, Cocos and Caribbean plates during the Miocene (13 to 2.7 Ma BP). The Nazca is still subducting under the Caribbean plate and the subduction zone forms a NE-SW trending chain of active volcanoes parallel to the Pacific Coast. The recent geological activity in Central America has resulted in considerable lithological diversity.

However, the geology of Barro Colorado Island is relatively simple and the lithological range is limited (Figure 2.1). The island is underlain by three formations: the Bohio dating back to the early Oligocene, the younger Caimito formation from the late Oligocene; and a later extrusive andesite cap (Ministerio de Comercio e Industrias, 1976; Woodring, 1958). The Bohio and Caimito each have two facies: volcanic and marine sedimentary.

The Bohio volcanic facies forms the northwestern part of the island and also stretches NS through the centre (Fig. 2.1). USGS (1980) shows the southern end of the NS outcrop widening to the west along the eastern section of the Wetmore Trail. This accords with the topography of that area. Only the upper 125 m of the estimated total formation thickness of 300 m are exposed on BCI (Woodring, 1958). The formation is variable, but the main Bohio rock type on BCI is agglomerate, which consists of basaltic clasts of all sizes (pebbles, cobbles and boulders) in a matrix of finer basaltic clasts (Woodring, 1958). Some of the clasts are very large, with boulders up to two meters in diameter.

The Bohio marine facies is interlayered with the agglomerate. It consists of greywacke sandstone of poorly sorted angular basaltic coarse sand in a fine-grained matrix containing feldspars and some quartz (Woodring 1958). On BCI the marine facies is subordinate and is not delineated separately.

The marine facies of the Caimito formation rests conformably on the Bohio formation. The outcrops on BCI expose only the lowest 100 m of the total 300 m thickness of the facies (Woodring, 1958). The main constituents are well sorted, tuffaceous and fossiliferous sandstones of varying grain-size. There are also subordinate interlayers of various types of calcareous and carbonaceous sandstones. Fossiliferous limestone occurs as lenticular beds between the sandstone layers, mainly in the northern outcrop (Woodring, 1958). Johnsson and Stallard (1989) emphasise the biogenic constituents and characterise the Caimito marine facies predominantly as foraminiferal limestone with abundant pelecypods. They describe a substantial detrital component of vitric volcaniclastic debris, plagioclase and quartz

There are two main outcrops of the Caimito marine facies on BCI, and these give rise to two distinct topographies and soil consociations. The more extensive is in the west and southwest of the island. The smaller northern outcrop is the central and eastern sections of the catchment of Lutz Creek between the limbs of the Lutz-Drayton fault system. The combination of volcanic and various sedimentary constituents makes the Caimito marine the most lithologically and pedogenetically diverse of the island’s geologies.

The volcanic facies of the Caimito formation underlies the eastern third of the island, with a thickness on the island of about 100 m (Fig. 2.2). The main constituents are basaltic agglomerates, which weather to give a bouldery regolith. There are also some greywacke beds, which vary with respect to size sorting. The Caimito volcanic facies abuts the Bohio volcanic formation on its western flank, separated by the Lutz-Drayton fault system (Woodring 1958).

There are intrusive and extrusive igneous rocks, which date back to the Oligocene and early Miocene. The most extensive is the andesite that caps the centre of the island (Johnsson & Stallard, 1989). In the west it rests on the Caimito marine formation; on its northern and western flanks it is in contact with the Bohio volcanic formation. It forms a flat, very slightly dipping plateau. The cuesta-like form is attributed to the gentle dip of the lava sheets down from NE to SW. The gentle dip is caused by a syncline, which plunges westward (Fig 2.2). The southern and western edges of the plateau drop down in short and moderately gentle connecting steps between a series of inextensive lower surfaces, the treads of which may relate to individual flow sheets. About 85 m of the flows are exposed on BCI (Woodring 1985). Elsewhere on BCI andesite is mapped as dikes and sills (Woodring, 1958). These are difficult to identify, as they are lithologically similar to the country rock, and both are substantially weathered. This unit has sometimes been designated as basaltic, but the presence of very small quantities of fine quartz grit in some soil profiles suggests a transitional intermediate-mafic lithology.

The general geological structure of BCI is a syncline which trends SSW-ENE and plunges westward (Fig. 2.2). There is an asymmetric anticline to the north, with its northern limb steeper than the southern. Its axis has not yet been located on the ground. The most obvious geological structural feature is the fault system that trends NNE-SSW across the centre of the island (Fig. 2.1). This is a sinistral strike slip fault with an eastward normal faulting component of a few dekametres. The left lateral movement, where the eastern block has been displaced northward relative to the western, has been estimated at 10 to 15 kilometres from observations off-island (Woodring, 1958). The fault bifurcates in the northern part of BCI, with the limbs enclosing the northern outcrop of the Caimito marine facies in the Lutz Creek catchment.

Regolith

The rocks of the island form only a limited lithological range of soil parent materials, as many of them, including most of the sedimentaries, are of volcanic derivation and of andesitic-basaltic composition. They weather to saprolites that are high in clays and moderately high in bases. There are no felsic rocks or significantly quartziferous, acidic or coarse-grained saprolites. However, the lithological uniformity should not be overstressed, as the Caimito marine calcareous sedimentary facies weathers to smectitic clays, in contrast to predominantly kanditic and oxidic saprolites from the other formations (Johnsson and Stallard, 1989).

There are few alluvial deposits on the island, the lowest lying of which appear to result from the increase in the regional base level following the flooding of the Gatun Lake
Some of the slope regoliths appear to be colluvial, emplaced by slopewash and creep. Stone sheets in the deeply weathered andesitic soils on the central dipslope/plateau are indicators of regolith mobility. The clasts are now soft, indicating that they have weathered in situ, as they could not have survived transportation in their present fragile state. This suggests that colluviation was more active previously than at present.
There are also colluvial soils around the fringes of the andesite. These have upper horizons that appear to have developed in shallow andesitic wash over lower subsoils that are residual from underlying marine sedimentary rocks.

The regolith derived from the Caimito volcanic facies in the east of the island contains many boulders. These give extensive boulders fields, especially on the upper parts of the dipslope, between the Barbour and Chapman trails. There are also many subsurface boulders and the soils of the Hood and Chapman classes have bouldery subsoils.