3-D VISUALIZATION AND CONTOUR MAP OF AN ANDEAN HINTERLAND BASIN BY GRIDDING MULTIPLE 2-D GRAVITY TRANSECT MODELS

3D views and contour maps provide accurate subsurface dimensions of a hinterland basin (e.g. Sibundoy Valley). Structural and geologic interpretations are derived from 3D shape of a basin. Here, we discuss the key points of the gravity method and modelling that enable a 3D visualization of an Andean hinterland basin.

Finding the gravity anomalies of the Sibundoy Valley (hinterland basin) was a challenge due to the complexity of isolating the basin gravity signal from the bulk of the Andean basement (regional signal). Also, the Andes show abrupt elevation changes over short distances in its topography, so the upward pull from the hills and the downward pull from the basin on the gravimeter was constrained using terrain corrections. We followed the Blais and Ferland (1984) terrain correction method that works well for gridded DEM elevation data. This method uses the gravity attraction of a right rectangular prism in each DEM cell with respect to a particular gravity station. To remove the regional signal, we obtained the International Gravimetric Bureau (BGI) dataset for the Colombian Andes. GRASS software enabled extraction of the gravity data with respect to a profile that covered the basin and the surrounding areas. The regional BGI Bouguer data curve is linear across the Sibundoy basin. The regional trend is then subtracted from the locally collected gravity data. This latter correction highlights the shorter-wavelength anomalies that are correlated to shallow geologic features (e.g. Sibundoy Valley).

The 3D visualization was created by a minimum curvature gridding method in Oasis Montaj. The data for the gridding method consists of the depth and distance of the basin from 2-D perpendicular structural gravity models, specifically collected on the intersection among the models. The contour map was created with the same information from the 3D visualization but using contour lines as the representation technique for elevation data. The models were constrained by different densities of the basin and lithologic geometries, to evaluate contrasting tectonic scenarios.

Even though, the discussed method was applied for an Andean geologic setting, a similar method could be applied to parts of the NC Piedmont area where the geologic mapping of the basement becomes difficult due to sedimentary wedge cover.