Morphological and Depositional Facies of Transgressive Dunefields in the Imbituba-Jaguaruna Region, Santa Catarina State, Southern Brazil C
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Journal of Coastal Research SI 39 673 - 677 ICS 2004 (Proceedings) Brazil ISSN 0749-0208 Morphological and Depositional Facies of Transgressive Dunefields in the Imbituba-Jaguaruna Region, Santa Catarina State, Southern Brazil C. T. Martinho†; P.C. F.Giannini†; A. O. Sawakuchi† and P.A. Hesp‡ †Instituto de Geociências, Dept. de Geologia ‡Geography and Antropology, Louisiana Sedimentar e Ambiental State University Universidade de São Paulo, São Paulo 227 Howe/Russell Geoscience Complex, 05508-900, Brazil Baton Rouge LA 70803-4105 USA [email protected], [email protected] [email protected], [email protected] ABSTRACT MARTINHO, C.T.; GIANNINI, P. C. F.; SAWAKUCHI, A. O. andHESP, P. A., 2006. Morphological and depositional facies of transgressive dunefields of the Imbituba-Jaguaruna region, Santa Catarina State, Southern Brazil. Journal of Coastal Research, SI 39 (Proccendings of the 8th International Coastal Symposium), 673 - 677. Itajaí, SC, Brazil, ISSN 0749-0208. This paper presents a morphological (process-form) and depositional (process-product) facies analysis of two types of active transgressive dunefields (facies associations) in the mid-south coast of Santa Catarina State. The Ibiraqüera dunefield (Imbituba) has parabolic geometry and about 5 kilometers of extension. It migrates inland and it's separated from the beach by a vegetated deflation plain. The influence of the vegetation on the dynamics of morphological facies in this dunefield is very common. The Grande do Sul dunefield (Jaguaruna) migrates parallel to the beach along tens of kilometers. It has a predominance of unvegetated morphological facies. When common facies to both dunefields are compared, the Grande do Sul's morphological facies are larger and their depositional facies are finer-grained. Two hypotheses can explain the differences between these dunefields. The first one involves a larger sand volume available to be transported by the wind at Grande do Sul beach: this sand volume comes probably from the adjacent inner shelf, which is wider and gentler on the Jaguaruna coast than on the Imbituba coast. The second hypothesis considers the angular relation between shoreline orientation and the effective wind direction. The maintenance of the eolian supply is favored in the beach-dunefield system that has an orientation more parallel to the prevalent wind from NE, i.e. Grande do Sul dunefield. The grain size differences are possibly related to the length of the dunefields: the tendence to finer and better sorted sediments in Grande do Sul dunefield than in Ibiraqüera dunefield can be attributted to greater length and longer transport in the former. ADDITIONALINDEX WORDS: Dunefield, Santa Catarina´s coast, eolian facies. INTRODUCTION METHODS In the active transgressive dunefields from the middle-south The depositional facies were individualized and described coast of Santa Catarina State (Figure 1), two different facies through trenches cut in each recognized morphological facies. associations can be recognized by their morphological aspects. When available, natural outcrops were described too. In These associations have been denominate respectively as distal morphological facies that have slipfaces, surface strike and dip and proximal, when the criterion is their relative position in the were systematically measured with the purpose to compare the depositional system, and as deflation and massive, when the modern bedform spatial orientation patterns with the criterion is the morphodynamic state (GIANNINI , 2002; Figure correspondent cross-stratification and bounding surfaces 2). The distal facies association predominates in the north from patterns, and then elaborate models to explain the mechanism Laguna county and presents smaller extensions (about 5 and preservation potential of these stratifications in the kilometers), with a deflation plain between the dunefield and geological record. The criteria used to identify and classify the the beach. This facies association must be related with a lower bounding surfaces were those established by BROOKFIELD influx/eflux ratio. The proximal facies association occurs (1977) and modified by KOCUREK (1981). mainly in the south from Santa Marta cape, and extends for The granulometric results refer to the mean diameter and many kilometers alongshore, without deflation facies. It has standard deviation calculated by the moments analythical been formed by the continuous pread of sand along many technique. The mean diameter is expressed in the nominal scale kilometers on the coast under a higher influx/eflux ratio. In both of WENTWORTH (1922) and the standard deviation values are facies associations, the dunefields migrate to SW, blown by the converted to the nominal sorting classes of FOLK and W ARD NE effective wind. (1957). The purpose of this paper is to characterize the morphological facies present in both dunefields and to MORPHOLOGICAL FACIES recognize the correspondent depositional facies based on grain size attributes, sedimentary structures and bounding surfaces. The identified morphological facies can be The aim is to relate the active processes in each facies morphodynamically separated in vegetation-influenced and association with their corresponding morphological and vegetation non-influenced. The facies with vegetation sedimentary responses, establishing the process-form and influence, from proximal to distal are nine: process-product relations. The chosen dunefields are located at Foredunes: are aeolian accumulations of sand in the Ibiraqüera's beach, Imbituba county, and at the Grande do Sul's vegetation, continuous alongshore, occurring on the inner beach, Jaguaruna county, representing the distal and the backshore (for intermediate/reflective morphodynamic beach proximal facies associations, respectively (Figure 1). types) or on the upper foreshore (the morphodynamic beach Journal of Coastal Research, Special Issue 39, 2006 674 Martinhoet al . 60o 66o AREA LOCALIZATION IN THE STATE o o 4 54 o 4 RR AP 48o 0o 0o o 42o PARANÁ 26 o AM o 72 PA 4 36o 4o MA CE RN 8o PI PE 8o AC TO PB ARGENTINA o AL SANTA CATARINA 27 RO o SE OCEAN 72 12o BA MT 12o o 66o 36 GO 16o 16o o RIO GRANDE o 60 28 MG DO SUL o 20 ES MS 20o SCALE SP 50 0 100 km RJ 0 300 600 900 km 24o 24o PR o 42 29o SC ATLANTIC o 28 28o RS 48o o o o o o o 32 32o 53 52 51 50 49 54o QUATERNARY COASTAL DEPOSITIONAL Ibiraqüera SYSTEMS Lagoon Ibiraqüera 28º10' Strand Plain System Beach Holocene Pleistocene IMBITUBA Mirim Barrier-Bar System Lagoon Guaiúba Holocene 28º20' Lagoonal System Holocene Imaruí Lagoon Eolian System Undifferentiated LAGUNA Sto. Antônio Lagoon Ji Beach 28º30' TUBARÃO N A OTHER UNTS Sta. Marta E Lagoon C O Quaternary undifferentiated continental sediments Grande do Norte Beach C I Santa Marta Cape T JAGUARUNA N Tertiary undifferentiated continental sediments A Garopaba L N T do Sul Lagoonl A 28º40' Pre-Cenozoic igneous, metamorphiic and 6km sedimentary rocks Grande do Sul Beach 49º00' 48º50' 48º40' Figure 1. Studied area localization, southern Brazil, mid-south coast of Santa Catarina State, Imbituba-Jaguaruna region. The quaternary coastal depositional systems are based on GIANNINI and S ANTOS (1994) map. type that predominates on the Santa Catarina's coast is (for active downwind margins of deflation plains and basins (similar dissipative) morphodynamic beach types). to foredune formation), or (ii) vegetation which has colonized Blowouts: are mixed (erosional-depositional) features, the lower stoss face of transverse dunes. They are formed during formed by wind erosion of preexisting sandy deposits followed oreverseffshore wind events. As the dune or dunefield migrates by downwind local redeposition. The morphology consists of a downwind, the gegenwalle ridge is left behind, and in case (ii), deflation basin delimitated by subparallel erosional walls that preserveings the upwind transverse dunefield edge geometry. close, toward the downwind side, into a U shaped depositional Nebkhas: are discrete vegetated sand mounds formed by lobe. eolian deposition in vegetation. Shadow dunes may form Parabolic dunes: are characterized by a U shape geometry. downwind of the nebkha. They display similar components to a blowout, but are Interdune plains: are almost flat areas, placed between the distinguished by having longer erosive walls or trailing ridges dunes, where deflation predominates over deposition. The or arms. interdune plains are characterized by constant moisture, Trailing ridges: These are of two types: the long arms of the periodic wetting and a dispersed cover of pioneer vegetation. parabolic dunes (above); and low, long narrow ridges formed Remnant knobs: are typically asymmetrical sand mounds, where vegetation colonises the outer margins of transverse partially vegetated, which occur in the middle of the dunefield. dunes. The inner side is eroded leaving a long trailing ridge They represent remnant parts of preexisting eolian deposits that behind as the dune migrates downwind (HESP , in press). They were differentially eroded. Their lee side is densely vegetated are parallel to the effective wind and, while forming, are while the stoss side is typically non-vegetated and characterized by having a depositional vegetated outerside and predominantly erosional. an erosive unvegetated innerside. Precipitation ridges: are long sand ridges that form where Gegenwalle ridges: are small sand ridges formed by sand sand “precipitates” or falls into vegetation along the margin(s) deposition in (i) deflation plain vegetation growing along