Mark J. Johnsson

First-Cycle Quartz Arenites in the Orinoco River Basin, Venezuela and Colombia

Modern first-cycle quartz arenites are forming in the Orinoco drainage basin by at least two distinct mechanisms. Common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur. In the Llanos (Andean foreland basin), extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains. In lowland portions of the Guayana Shield, long soil mineral residence times are produced by very low erosion and transport rates, resulting from low relief and tectonic quiescence. Both weathering intensity and the duration of weathering are important in the modification of sand composition. First-cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences. Furthermore, although tectonic setting is of great importance in determining sand composition, modifications during transport and deposition can overprint the effects of the tectonic regime. In extreme cases, such as first-cycle quartz arenites, the tectonic signal may be virtually obliterated. Many criteria used in the past to discriminate between first-and multi-cycle quartz arenites are invalid when applied to the quartz arenites of the Orinoco drainage basin. At present, the only unambiguous criterion is that the presence of sedimentary lithic fragments and syntaxial quartz overgrowths is clearly indicative of a multi-cycle origin for at least one component of a sand.

Controls on the composition of fluvial sands from a tropical weathering environment: sands of the Orinoco River drainage basin, Venezuela and Colombia

Fluvial sands in the Orinoco River drainage basin fall into three main compositional groupings: (1) sands of subarkose and arkose composition, from high-relief parts of the Guayana Shield, where crystalline rocks are exposed; (2) sands of litharenite and sub-litharenite composition from the active orogenic belt at the western and northwestern margins of the drainage basin, and in those parts of the Llanos (Andean foreland basin) proximal to the mountain belt; and (3) sands of quartz-arenite composition, widespread throughout the remainder of the basin. Multicycle sands of quartz-arenite composition are produced from platform cover on parts of the elevated shield and from uplifted foreland-basin fill in the eastern Llanos. First-cycle sands of quartz-arenite composition are produced from granitic rocks on low-relief regions of the Guayana Shield and from reworked Holocene alluvium in parts of the western Llanos distal to the orogenic terranes. Erosion in the Orinoco River drainage basin may be described in terms of transport-limited and weathering-limited denudation regimes. In active orogenic terranes and in parts of the elevated shield, transport processes can remove weathered material as rapidly as it is produced by chemical weathering. Thin soils and short soil-mineral residence time result in sands that are incompletely chemically weathered and accurately reflect source-rock composition. In the orogenic terranes, subtle variations in source-rock lithology are preserved in sand composition. In contrast, in low-relief parts of the Guayana Shield and on flat erosion surfaces of the upland shield, weathering exceeds the rate at which transport processes can remove weathered material. Thick soils accumulate, soil-mineral residence time is long, and detritus is highly altered chemically. On much of the lowland Guayana Shield, upper soil layers consist of nearly pure quartz sand that erodes to produce first-cycle fluvial sand of quartz-arenite composition. Chemically weathered orogenically derived sand enters the Orinoco River on the left bank, while feldspathic shield-derived sand enters on the right bank. This geometry is responsible for the nearly total lack of longitudinal variation in sand composition along the 1,400-km length of the Orinoco River mainstem. Except in the upper 100 km of the Orinoco River mainstem, cross-channel heterogeneity in sand composition is also modest. Nevertheless, a weighted linear least-squares modeling approach suggests that sand moves down the Orinoco River mainstem in distinct pulses, perhaps corresponding to times of accelerated erosion in headwater regions. Because rivers that head in the orogenic terranes and traverse the Llanos contribute more than 99% of the sand in the lower Orinoco River mainstem, the composition of this sand is dominated by chemically weathered sands from the Llanos. The Orinoco River—the third largest river in the world—delivers first-cycle sands of quartz-arenite composition to its delta.

Physiographic Controls on the Composition of Sediments Derived from Volcanic and Sedimentary Terrains on Barro Colorado Island, Panama

ABSTRACT The phenomena that control sand composition are complex, dynamic, and interlinked. Although source-rock lithology is of critical importance in determining the allowable range of sand compositions, chemical weathering may overprint the source-rock signal substantially. In the humid tropical environment of Barro Colorado Island, Panama, source rocks of similar lithological character produce sediments of differing compositions in different physiographic environments. Moreover, sediments derived from source rocks of dissimilar mineralogical composition but produced on similar physiographic terrains may be nearly indistinguishable. Physiography influences the composition of sediments released from the source areas by controlling the duration of chemical weathering. On relatively flat terrains, rates at which loose solids are generated by weathering may exceed rates at which slope processes can remove this material. Soils are exposed to the weathering environment for long periods of time and are strongly leached. Consequently, weathering products are cation poor and contain relatively immobile elements. Incompletely weathered detritus is not abundant and is greatly enriched in chemically resistant phases. On steeper slopes, potential solid erosion rates exceed weathering rates. Residence times of minerals within soils are short, resulting in weathering products that are cation rich and contain an abundance of uns able phases. On Barro Colorado Island, fluvial sediments are dominated by authigenic particles composed of secondary weathering products. Although the stability of many such particles in larger transportational and depositional systems and in the diagenetic environment has yet to be assessed, they may be more important constituents of fluvial sediments in tropical environments than has been previously recognized.

Global distribution of late Paleogene hiatuses

Six global late Paleogene hiatuses (PHa to PHe) have been identified from deep-sea sequences. These hiatuses occurred at the middle/late Eocene boundary, late Eocene, Eocene/Oligocene boundary, late early Oligocene, late Oligocene, and Oligocene/Miocene boundary horizons. Paleodepth distribution of hiatuses shows hiatus maxima characterized by major mechanical erosion below 4800 m, at mid-depth between 2000 and 3000 m, and in shallower water above 1600 m paleodepth. The geographic distribution and paleodepth of these hiatus maxima suggest that flow paths of major water masses and currents are the principal cause. Widespread short hiatuses due to carbonate dissolution or nondeposition occurred primarily during global cooling trends or climatic instability and appear to correlate to sea-level transgressions or onlap sequences. These hiatuses may have been caused by basin-shelf fractionation of carbonates.

Weathering processes and the composition of inorganic material transported through the orinoco river system, Venezuela and Colombia☆

Abstract The composition of river-borne material in the Orinoco River system is related primarily to erosion regime, which in turn is related to tectonic setting; especially notable is the contrast between material derived from tectonically active mountain belts and that from stable cratonic regions. For a particular morpho-tectonic region, the compositional suites of suspended sediment, bed material, overback deposits, and dissolved phases are fairly uniform are are typically distinct from whose of other regions. For each region, a consistent set of chemical weathering reactions can be formulated to explain the composition of dissolved and solid loads. In developing these formulations, erosion on slopes and storage of solids in soils and alluvial sediments are important considerations. Compositionally verymature sediment is derived from areas of thick soils where erosion is transport limited and from areas where sediments are stored for extended periods of time in alluvial deposits. Compositionally immature sediments are derived from tectonically active mountain belts where erosion is weathering limited. Weathering-limited erosion also is important in the elevated parts of the Guayana Shield within areas of sleep topography. Compared to the mountain belts, sediments derived from elevated parts of the Shield are more mature. A greater degree of chemical weathering seems to be needed to erode the rock types typical of the Shield. The major-element chemistry and mineral composition of sediment delivered by the Orinoco River to the ocean are controlled by rivers that have their headwaters in mountain belts and cross the Llanos, a region of alluvial plains within the foreland basin. The composition of sediments in rivers that drain the Shield seems to be established primarily at the site of soil formation, whereas for rivers that drain the mountain belts, additional weathering occurs during s episodes of storage on alluvial plains as sediments are transported across the Llanos to the main stem of the Orinoco. After mixing into the main stem, there seems to be little subsequent alteration of sediment.