Fluvial-eolian interaction systems: examples of modern and ancient deposits from northwestern Argentina

TRIPALDI, A.; LIMARINO, C.O.; NET, L.I.,

Belem, Brasil

Congreso; 3rd Latin American Congress of Sedimentology; 2003

International Association of Sedimentologists

<!-- /* Style Definitions */ p.MsoNormal, li.MsoNormal, div.MsoNormal {mso-style-parent:""; margin:0cm; margin-bottom:.0001pt; text-align:justify; mso-pagination:widow-orphan; tab-stops:14.2pt; text-autospace:none; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:ES-AR;} p.MsoBodyTextIndent2, li.MsoBodyTextIndent2, div.MsoBodyTextIndent2 {margin:0cm; margin-bottom:.0001pt; text-align:justify; text-indent:14.2pt; mso-pagination:widow-orphan; tab-stops:14.2pt; text-autospace:none; font-size:10.0pt; font-family:"Times New Roman"; mso-fareast-font-family:"Times New Roman"; mso-ansi-language:EN-US;} @page Section1 {size:612.0pt 792.0pt; margin:70.85pt 3.0cm 70.85pt 3.0cm; mso-header-margin:36.0pt; mso-footer-margin:36.0pt; mso-paper-source:0;} div.Section1 {page:Section1;} --> Eolian sedimentation is a basic component of many continental basins, which can form from thick cross-bedded sandstone sequences to thin intercalations in fluvial and lacustrine facies. In transitional settings, the interplay between fluvial and eolian processes produces a particular environment known as "fluvial-eolian interaction system" (Langford, 1989). Although these deposits are common features in desertic areas they have not been studied in detail up to recent times (Langford and Chan, 1989; Trewin, 1993; Sweet, 1999; Newell, 2001; Veiga et al., 2002). Fluvial-eolian interaction environment has a complex regime where fluvial-deposited sediments are sporadically formed and afterward exposed to wind reworking. The aim of this contribution is to present two examples of mixed fluvial-eolian facies: a) modern deposits located along the Guandacol valley (La Rioja province, Argentina), and b) Miocene redbeds of the Upper Member of the Miocene Vinchina Formation (Western Precordillera, La Rioja province, Argentina). The intermontane Guandacol valley (Fig. 1a) results a braidplain of ephemeral channels with interchannel areas dominated by several micro- and meso-scale eolian bedforms (Fig. 1b; Tripaldi, 2002). Based on remote sensing, aereal photomosaics, field observations and bedform association and distribution, two subenvironments have been identified: 1) fluvial channel areas, represented by the main Guandacol River channel and its minor tributaries and 2) interchannel areas, dominated by eolian processes. Fluvial channel areas are covered by poorly-sorted sediments organized as gravel-rich lags and small sandy bars together with common mud drapes and desiccation cracks. Channels have an ephemeral regime, remaining dry most of the year. For this reason fluvial sediments are reworked by winds forming discontinuous sheets of eolian ripples and small sand shadows. On the other hand, interchannel areas are mostly covered by very well- to well-sorted fine sands of eolian origin. A great variety of wind-laid bedforms such as eolian ripples, sand shadows, zibars, gozes, protodunes and small barchanoid dunes are shaped in these areas, producing an irregular and low topography, similar to that formed in sand sheets. Scattered fluvial deposits appear as gravel lags and mud drapes among these eolian bedforms. A disperse low vegetation is also present in these areas. The Vinchina Formation (Turner, 1964) is a thick redbed sequence of Miocene age that crops out in the Western margin of the Sierras Pampeanas (Fig. 1). A detailed analysis of its architectural elements allowed interpretating different fluvial styles in this sequence (Tripaldi et al., 2001). To the top of the unit, some sparse fluvial-eolian interaction intervals associated to ephemeral stream facies have also been recognized (Tripaldi, 2002). These intervals have been divided in fluvial and eolian deposits (Fig. 1c). Fluvial deposits comprise channel and floodplain units. The former are made up of amalgamated lenticular bodies of medium- to fine-grained cross-bedded sandstones together with massive and cross-bedded intraformational conglomerates, in most cases showing erosive bases. Floodplain units are formed by rippled- or horizontally-laminated fine- to very fine-grained sandstones and thinly interlayered massive mudstones, which commonly exhibit bioturbation and desiccation cracks. Eolian deposits are formed by matrix-free and well-sorted fine- to very fine-grained sandstones with a characteristic open-packed fabric. They show very thin horizontal or low angle cross-bedded lamination, where inverse graded lamina is a common feature at both outcrop and microscopic scale. High-angle cross-bedded sandstones are less frequent. They can form up to 1 meter-thick tabular bodies, whose foresets sometimes also exhibit inverse- graded laminae. The here described interval at the top of the Vinchina Formation is interpreted as ephemeral fluvial channels within muddy and sandy floodplains that intertongue with wind-laid deposits, thus reminding the modern Guandacol valley depositional environment. From the hydrocarbon reservoir potential point of view, mixed fluvial-eolian systems constitute complex reservoirs, generally strongly controlled by facies distribution (North and Prosser, 1993). Total optical porosity measurements in sandstone samples of the Vinchina Formation using petrographic image analysis (PIA) reveal significant variations between fluvial and eolian facies (e.g. 3% and 10%, respectively). Hence, the two analyzed fluvial-eolian interaction examples constitute useful analogues to help with the accurate identification of even volumetrically minor wind-laid facies interbedded in the fluvial record. This could have a significant impact during reservoir characterization in productive areas of similar depositional characteristics.