Analysis of Scallops in Gomantong Caves, by GIS Processing of 3D
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Analysis of scallops in Gomantong Caves, by GIS processing of 3D terrestrial laser scanner data Joyce Lundberg1, William Carroll1, Warren Roberts2, Donald A McFarlane2, Manfred Buchroithner3, and Guy van Rentergem4 Afliation: 1Carleton University, Ottawa, Canada 2Te Claremont Colleges, California, USA 3TechnischeUniversität, Dresden, Germany 4Koningin Astridstraat, Deinze, Belgium Abstract Gomantong Caves, occupying much of the isolated karst tower of Gomantong Hill, Sabah, Borneo, are relict phreatic caves (now famous for their huge populations of bats and swiflets), with no evidence of any vadose phase. Here we explore the speleogenesis by analysing the phreatic scallop remnants to determine former fow direction and thus hydraulic gradient. Data, at centimeter- scale resolution, were acquired from 3D terrestrial laser scanning using a FARO3D TLS. Eight populations of scallops were studied. Scan data were extracted using SCENE LT sofware, and imported into ArcGIS. For each surface (3-4 m2 in area), digital elevation models were produced of hill-shade and of slope. Cross sections highlighted the scallop asymmetry and thus direction of water fow. Te cave is largely made up of dip tubes tilting downwards from the entrances at an angle of ~22° and opening into large central passages. Surfaces in Simud Hitam (Black Cave) are strongly modifed such that the original scallops are ofen destroyed. Scallops in Simud Puteh (White Cave) are generally well preserved. Analysis of the scallops indicates that water fow in these phreatic passages had been upwards in the dip tubes and outwards towards the entrances. Tis implies a rising water source from depth, and a distant recharge source. Keywords: 1. Introduction In an attempt to elucidate the original speleogenesis and hydrology, we mapped and measured the remnants of phre- Gomantong Hill (5.51° N, 118.06° E) is an isolated limestone atic scalloping on the cave walls and ceilings using 3D ter- outcrop on the food plain of the Kinabatangan River, ~30 km restrial laser scanning. Several of the scalloped surfaces can south of Sandakan, NW Borneo, that encompasses a sizeable be seen clearly in the cave. However, many are far too high cave system (McFarlane, 2013). Gomantong Caves include to reach by conventional caving techniques and many cannot the lower level Simud Hitam (or Black Cave, with the tour- be seen under normal lighting conditions – hence the unique ist boardwalk) and the upper level, less accessible, Simud advantage of 3D scanning data: information can be acquired Puteh (or White Cave) (Figure 1a). Te caves are famous for that otherwise would be unattainable. In addition, we exploit their bird’s nest harvesting and the huge population of bats. the extraordinary density of data to make measurements that Tey are developed in the ~300 m-thick outcrop of Goman- in the feld may be very time-consuming and considerably less tong Limestones, of upper Oligocene to lower Miocene age, accurate. Taking further advantage of modern technology, we made up of isolated inliers of mainly reefal limestone, lying married the 3D point-cloud data with GIS techniques to pro- unconformably on, or interfngering with, sandstones and duce maps of slope for each site. Scallop asymmetry allows the shale turbidites. Landscape development and speleogenesis reconstruction of former fow directions and scallop dimen- is presumed to have occurred in the last couple of million sions allow calculation of former fow velocity (Curl, 1974). years since Late Pliocene tectonism tilted and folded the beds. Together with passage dimensions from the point-cloud data, Gomantong Hill is a synclinal feature, with dips of ~5-25° and we can then produce estimates of former hydraulic gradients in the region of the cave the dip is 20-30° to the NW (Wilford, 1964). and discharges. Te cave is developed along bedding planes and steeply 2. Methods inclined joints, but in places shows substantial modifcation Tree-dimensional laser scan data were collected during that ofen disguises the original shape. Te bedding plane pas- two feld sessions, in August 2012 and 2014, using a FARO sages are typical phreatic dip tubes, elliptical in cross section Focus3D instrument (details in McFarlane et al., 2013). Visu- with scalloped walls and ceilings, the dip at ~20°, but variable. alization of the point cloud data was done with Faro SCENE Te joint passages are high with very complex arch-shaped LT sofware. cross sections. Evidence for the former phreatic origin is now ofen hard to detect, many of the surfaces having been signif- Areas of potential interest were identifed in the feld and cantly altered by post-speleogenetic modifcation (Lundberg from the point cloud data. Two types of scallops are appar- et al., 2012). Te caves are now fully drained, with only one ent in the cave, the more common being scallops from former minor in-cave stream, and appear to have been drained rap- fuvial current activity (the features of interest here), and the idly, no vadose stage being apparent. less common being air scallops from ongoing condensation corrosion by diurnally-exchanging currents of humid air. Te Proceedings of the 17th International Congress of Speleology 285 Figure 2. Example of output from ArcMap. Tis site is from the main entrance to Simud Puteh, the upper level cave. A. DEM showing Figure 1. A. Photograph of 3D print of lower cave, Simud Hitam hillshades. B. SlopeMap showing steep (red), moderate (yellow) and (Black Cave) in orange plastic, and upper cave, Simud Puteh (White gentle (green) slopes. C. Cross section. Flow direction is indicated by Cave) in white plastic. Te two caves are shown in correct juxta- heavy arrows position. Te scale is approximate. B. Simplifed survey of the caves showing paleo fow directions deduced from scallop asymmetry. Te velocities were calculated using the equations from Curl upper and lower caves are separated to show passages more clearly. (1974) in an Excel spreadsheet from Woodward and Sasowsky (2009). types can usually be distinguished by their location, morphol- ogy and dimension: air scallops are generally confned to cave Cross sections of passages were produced with FARO Scene entrances, and are usually larger, shallower and less sharply LT, and the outlines traced using CorelDraw. Passage dimen- defned than fuvial scallops. We chose only features that we sions and cross sectional areas were measured using ImageJ. are confdent represent fuvial features. Together with the velocity data, this allowed calculation of paleo-discharge. Seven sites with areas of clear scallops were chosen. Te point- cloud data were exported as ASCII points, and imported into ArcMap. Tey were then converted into: 3. Results An example of the output from ArcMap is shown in Figure 2. 1. Digital Elevation Models (DEMs) to show hillsides (grey- Te DEM shows the scallops in greyscale, and the SlopeMap scale images) shows red, yellow, and green shading representing, respec- 2. Slope Maps (images colour-coded for slope) tively, steep (upstream sides of scallops), moderate and gentle slopes (downstream sides). Flow direction is indicated by the 3. Cross-sectional profles to demonstrate asymmetry of form heavy arrows. Flow directions from the seven sites were plot- ted on the simplifed cave survey (Figure 1b). Te upper-level Scallop lengths were measured from the ArcGIS output but cave has some seven clearly distinct passages, of which we also, in order to give larger sample sizes, from screen-captures were able to study fve. Te lower-level cave, largely made up of FARO Scene visualization imported into ImageJ. Flow of a complex chamber (which we hypothesise has been cre- 286 Proceedings of the 17th International Congress of Speleology ated by amalgamation of several formerly distinct phreatic tubes (Lundberg and McFarlane, 2012), has few clear passages and scallops could be measured in only two areas. Some parts of the cave yielded no paleo-fow data because they had no detectable scallops or scallops so severely modi- fed as to preclude measurement. Surfaces have been modifed by a variety of processes, most related to biogenic activity and associated condensation corrosion, and, in the large cham- bers, by roof collapse. Our results show that all passages studied show paleo-fow rising in the tube, uphill against the direction of dip, and all generally towards the east (Figure 1b). Flow has shifed towards the north in the lowest-level outlet, the main entrance to Simud Hitam, Black Cave. Examples of passage cross sections are shown in Figure 3. Flow velocity, calculated from Sauter means of scallop lengths (n = 25-50), and discharge, calculated from passage cross sec- tional areas and velocity, are shown in Table 1. 4. Discussion Terrestrial laser scanning is increasingly being employed in technically-difcult and complex caves (e.g., Buchroithner and Gaisecker, 2009). Terrestrial laser scanning at Goman- tong has now imaged virtually all of Simud Hitam and Simud Puteh. Te size of the point-cloud dataset creates signifcant sofware processing challenges but these can be addressed by point sub-sampling, and by breaking the data set into sections for separate processing in ArcMap sofware. Figure 3. Examples of passage cross sections. Te modern cross section is shown as a solid line. Te reconstructed former phreatic Te results of this analysis of scallop asymmetry show a pat- surface is shown as a dotted line. Te upper-level cave passages are tern of fow, for all sites with measurable scallops, that rises largely unmodifed. from depth, fowing upwards and generally eastwards, in the direction of the modern coastline. Many classic phreatic caves that artesian fow may have been important, the non-lime- show looped sequences of dip tubes and lif tubes (e.g., Ford stones rocks and perhaps fault zones acting as aquitards. As and Williams, 2007, p. 225). None of the passages in Goman- base level was lowered, the outlets shifed, fnally emerging tong shows descending fow.