Some Minor Features (Tafoni, Cavernous Forms) in the Granite Terrains of Los Riojanos (Pampa De Achala, Sierra Grande De Córdoba, República Argentina)
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Cadernos Lab. Xeolóxico de Laxe ISSN: 0213 - 4497 Coruña. 2008. Vol. 33, pp. 83 - 99 Some minor features (tafoni, cavernous forms) in the granite terrains of Los Riojanos (Pampa de Achala, Sierra Grande de Córdoba, República Argentina). Geometric and morphologic properties UÑA ALVAREZ, E. De1 and VIDAL ROMANÍ, J.R.2 (1) Área de Geografía Física-Ourense (Universidad de Vigo). Pabellón 2-1º. Campus Universitario As Lagoas. 32004-Ourense. E-mail: [email protected] (2) Instituto Universitario de Geología (Universidad de A Coruña). Campus Universitario de Elviña. 15071-A Coruña. E-mail: [email protected] Recibido: 1/11/2007 Revisado: 8/4/2008 Aceptado: 20/7/2008 Abstract Several minor cavernous features can be well differentiated in the granite terrains of Los Riojanos (Pampa de Achala, Sierra Grande de Córdoba, República Argentina). The cavernous morphology always appears related to a well-defined discontinuity rock system, which is determined by the endogenous massif history. Here, all the observations about cavern-like fea- tures, usually called tafoni, are located near intrusive border of outcrop. The samples of this work were recorded from a recent research on granite landscape. Their analyses can provide significant information to explain the geomorphic evolution of the massif. Their geometric and morphologic nature analysis is the first step in order to clarify what the morphologic and evo- lutionary keys linked to the genetic conditions towards a better landform explanation are. The measurement system and their three-dimensional projection are only previous information to obtain the summarized data (volume). The values distribution and their fit goodness are also the previous step for a suitable statistical processing. Considering the configurative aspects and the relationships with the host rock forms, we can advance towards the knowledge of these granite landforms. Key words: tafoni, cavernous forms, endogenous forms, granite landforms, granite lands cape, Argentina 84 Uña Álvarez and Vidal Romaní CAD. LAB. XEOL. LAXE 33 (2008) INTRODUCTION work deals with the minor granite landforms usually known as tafoni or cavernous forms, The granite landscape is a puzzling where although they have been termed with another the diverse components or landforms are inter- names in the geomorphologic research (UÑA dependent. Their generation and development ÁLVAREZ, 2005). They result from a hollow- pathways derive from a multi-feedback set of ing out process that commonly can be different signs subject to contingency along observed on the granite landscape underside the time (THOMAS, 2001). At a local scale, the blocks or boulders and into the rock walls, the study of one landform typology analyzes always related to the rock discontinuity system their convergent or divergent behaviour on (TWIDALE and VIDAL ROMANÍ, 2005). some rock masses. This behaviour is interpret- Therefore, the typology of all these cavities ed in an evolutionary context from a granite can provide significant information about the outcrop where a set of endogenous and exoge- genetic and evolutionary landscape events. nous events can define and model the rock This study deals with some tafoni features in units (VIDAL ROMANÍ et al., 2006). This “Los Riojanos” (Pampa de Achala, Argentina). Fig. 1 Location of Sierra Grande and the study area (Google Earth). CAD. LAB. XEOL. LAXE 33 (2008) Some minor features 85 Photo 1. General view of the granite landscape in Los Riojanos. Photo 2. Particular view of several granite cavernous forms. 86 Uña Álvarez and Vidal Romaní CAD. LAB. XEOL. LAXE 33 (2008) STUDY AREA placed between 1,686 and 1,819 m a.s.l. On the whole, the generation of granite landforms All the recorded cavities are located in the in the study area (a tectonic relief) starts from geographic unit “Sierras Pampeanas”, north- the endogenous events related to the intrusive western region of Argentina, concealment in process; their linked patterns appear in the Sierra Grande de Córdoba (cerro Champaquí, periphery near the former intrusive border of 2.790 m a.s.l.). The Sierra Grande de Córdoba the magmatic body where it is defined the is a granite massif intruded between the sampling station for this research (VIDAL Chaco-Paraná basin and the Pre-andina range. ROMANÍ et al, 2007). The boundary research concerns their high plains of “Pampa de Achala”, in the western SAMPLING AND WORK METHODS area (Fig. 1), declared Natural Hydrological Reserve in 1995. From the bio-geographic In order to collect a significant statistical viewpoint, the Pampa de Achala is a particular sample, the fieldwork has sustained two main eco-region of the Sierras and Bolsones system selection criterions of the forms. Only just with a sub-tropical dry climate. The maximum were collected the forms which hostess block value of annual precipitation is 200 mm that is remained “in situ” (not moved or not split). In concentrated in the summer months. The addition, the sampling (n=25) was limited to mean annual temperature is 14ºC but during the greatest forms. The measurement of the the winter months has been recorded -5ºC; the different cavities form is really free-scale in temperature variation along the day usually nature; thus, it permits a selective analysis shows a wide range from 5ºC to 35ºC in a nor- which can be used to establishe a hierarchical mal summer day. From the geomorphologic setting. But it is clear also that this research viewpoint, the Pampa de Achala is a horst approach simplifies the morphological fea- form which has risen during the post- tures. Because of that in the present work the Mesozoic times; these events have affected a measures are detailed from a two-dimensional previous etch surface (CARIGNANO et al, plane to a three-dimensional volume. In accor- 1999) with a multistage time sequence. dance with our recent related works (UÑA The research sample comes from “Los ÁLVAREZ, 2004; UÑA ÁLVAREZ and Riojanos” (in forward LR), extending VIDAL ROMANÍ, 2006), the form attributes between latitude 31º28´40´´S - 31º29´15´´S or the studied variables taken into account deal and longitude 64º55´00´´W - 64º55´55´´W with the current cavities design. In this context (Photos 1 and 2). The studied landforms are primary and secondary data were managed. Photo 3. Basal feature. Photo 4. Lateral feature. CAD. LAB. XEOL. LAXE 33 (2008) Some minor features 87 Photo 5. LR8 - Baldachin form very developed in volume. The primary data of the landforms were forms show only lateral ringlets supports recorded by the fieldwork (August 2006). more or less differentiated; and occasionally, They included features related to the location, it can be observed a hanged granite pillar the shape and the qualitative properties of the which remains of the previous support. The cavities. The primary measures inside a cavity secondary numeric data of the landforms were were the dimension of the main horizontal the evacuated rock volume which was noted axis (y, x) as well their perpendicular vertical in cubic metres. This volume datum required axis (z). The values of these measures (unit = the previous approximation about the three- metres) were stated on a three-dimensional dimensional shape for use an adequate calcu- space. The projection in a Cartesian coordi- lation formula. So, the geometric bodies deter- nates provided some geometric properties mined by the measured axes and their projec- which were very significant to characterize an tion on a three-dimensional space represent a empty space inside a rock unity, in their size basic analysis tool. and their shape (Fig. 2); this field information All numeric data were object of a statisti- was basic to know the opening and deepening cal processing. First, we consider some sta- of the cavities. Moreover, during the field tistical descriptive measures. Between them, research it was especially important to identi- two standardized stats were used to explore if fy the nowadays support system of the cavities our sample comes from a normal distribu- (Fig. 3): some forms preserve a central granite tion: the skewness which looks for lack of pillar on the adjacent rock plane whereas other symmetry in the data distribution; and the 88 Uña Álvarez and Vidal Romaní CAD. LAB. XEOL. LAXE 33 (2008) kurtosis which reveals if the shape is more (based upon comparing the distribution peaked or flatter than the normal distribution. quantiles) and Kolmogorov-Smirnov test The best distribution fit of our data was full (based upon comparing the cumulative distri- determinated by the Shapiro-Wilks test bution frequencies). Fig. 2 Ellipsoidal, spheroid and other geometric references. Second, the data processing was stick to robustness with non-parametric stats. The robust measures are the most adequate descriptors for skewed or non-normal shape distribution of data. These statistics are not sensitive to the extreme values (outside values) which should be never excluded in the geomorphic research. In the box plot of the data, the median is marked by the centre vertical line; the lower and upper quartiles comprise the edges of the central box. The analysis ends with the volume data cluste- ring for detecting stage-groups of the cavities. Here, the cluster computes normalized Euclidean distances. The median linkage method uses the median value of all observa- tions as the reference point for distances bet - ween pairs of cases. In order to abstract the size and the form of the found cavities were drawn Fig. 3 Supports and rock pillars conditions. all cross-sections along the (x) axis (Fig. 4). CAD. LAB. XEOL. LAXE 33 (2008) Some minor features 89 'm Fig. 4 Tafoni cross-section along (x) axis. 90 Uña Álvarez and Vidal Romaní CAD. LAB. XEOL. LAXE 33 (2008) GEOMETRY AND MORPHOLOGIC shape of the cavities can be usually represent- FEATURES ed as a part of an ellipsoid (Tables 1.1 & 1.2). Generally, the geometric variation of these The geometry reference appears in two ellipsoidal cavities (mid-ellipsoidal or quarter- ways defined by the main development of the ellipsoidal) implies several specific values cavities into the hostess block: basal (Photo 3) from an oblate to a spindle-shaped geometric or lateral (Photo 4).