Reports: AC8
45910-AC8 Modern Andean Foreland Basin Lakes: Keys to Deciphering Ancient Foreland Basin Lacustrine Stratigraphy
This project made significant progress during the past year in defining the depositional characteristics of several representative foreland basin system lakes in Argentina and Brazil, and in placing those findings in a broader context of modern and ancient foreland basin lake deposystems. Major focal deposystems this year included modern piggyback(wedgetop), broken foreland, and backbulge lake deposystems. Shallow, expansive lakes housed within piggyback basins are important and conspicuous components of many retroarc foreland basin systems. Over geologic time scales these deposystems may develop into hydrocarbon source intervals. The Quenca de Pozuelos, a thrust-fault bound piggyback basin in northwestern Argentina, provides an opportunity to assess lacustrine and other non-marine depositional patterns in the arid Andean Puna. The locus of modern deposition is focused along the basin’s axis, where an endorheic lake is fed longitudinally by rivers and laterally by ephemeral alluvial fan complexes. The lake, Laguna de los Pozuelos, is large (~ 105 km2), slightly alkaline (pH ~ 8.65), shallow (< 2 m) and oligotrophic. Surface sediment samples collected from the lake indicate total organic carbon values increase toward the lake center, reaching a maximum of ~2.6 wt %. In contrast, total inorganic carbon values increase towards the lake margins, where values commonly exceed ~1.5 wt %. Hydrogen index values range between 55-190 mg HC / gm TOC, suggesting sedimentary organic matter is dominated by terrestrial sources and may be gas prone. This is also supported by *13Cbulk organic matter data, which is more enriched than in the other lake systems analyzed and also shows a mostly terrestrial origin. Typical organic matter accumulation in the wedgetop is highly oxidized Type III kerogen. Pb-210 data indicate that recent sedimentation rates in the basin are relatively rapid, approaching 0.15 cm/yr. Short (3-6 m) hammer cores collected from lake margin, delta, and axis environments exhibit a wide range of lithofacies, including sheet flood and turbidite sands, finely-laminated, organic-rich sapropels, massive, ostracode-rich or pyrite-rich silty clays and mud-cracked sandy clays. Facies transitions are in many instances abrupt, and are interpreted to be controlled by water level fluctuations associated with late Quaternary climatic and tectonic change.
Studies of Laguna Mar Chiquita (Argentina), the largest broken foreland lake of the Andes, have focused on depositional patterns in organic matter and associated sedimentology. Mar Chiquita displays intermediate TOC (3-6%) and TN (0.3-0.8%) values relative to wedgetop (low) and backbulge (high) systems, relatively enriched *13Cbulk organic matter, indicative of a mixed land plant/algal origin, and intermediate *15Nbulk organic matter. TOC and biogenic silica are both generally focused in deeper parts of the lake, although the relationship is not exact.
Lakes Mandiore, Gaiva and Baia Vermelha of the Brazilian Pantanal and lakes of the distal Chaco (Argentina) have been the focus of backbulge basin investigations. Most deposystems in this zone are expansive, seasonally flooded wetlands, with lakes mostly restricted to shallow basins adjacent to major rivers. In comparison with wedgetop and broken foreland lakes, these backbulge lakes display a wide range of productivities, ranging from productive lakes dominated by algal organic matter deposition to lakes with lower production and more allochthonous plant matter accumulation. All backbulge lakes display relatively depleted *13C and *15N in their bulk organic matter compositions, intermediate values of HI and highly variable OI values (all Type II).
Lakes of the Andean foreland system are uniformly shallow (<10m) and typically bowl-shaped in morphology. Many parts of the foreland basin system (especially in the foredeep) are overfilled, with only seasonal wetlands as standing waterbodies. Formation of somewhat deeper foreland basin lakes (evident from the stratigraphic record) appears to be contingent on appropriate watershed lithologies. These include highly resistant to weathering and/or comprising significant proportions of soluble bedrock, which can be removed in solution from the deposystem without contributing to overfilling.
Ongoing work resulting from PRF support is currently focused on the interpretation of short cores from the various lake types, to determine sediment accumulation rates, vertical facies stacking and parasequence characteristics..
