TOPIC DESCRIPTION
In the course of fieldwork the writer has encountered a number of geologic phenomena which, taken at face value, will have been interpreted wrongly. This note describes such features which comprise morphological, structural, as well as sedimentological criteria.
EXAMPLES
1. Triangular facets represent a geomorphic phenomenon which is generally assumed to indicate remnants of faultplanes. The facets occur as truncated spur ends and mark an intermediate stage of erosion of the fresh fault scarp to the faultline. The Lembang fault scarp is partially facetted, especially near Tjisarua approximately 15 km NNW of Bandung.
Triangular facets, however, were also observed on steep dipslopes without faulting near Tomo, West Java. A facetted scarp is displayed on steeply inclined dipslopes of volcanic arenaceous and rudaceous strata of the so called Tjilutung beds (Middle Pliocene); see figure 1. These facets have an essentially similar development like those on fault scarps. After the steeply inclined bedding planes have become exposed, continuous gullying has
carved the dislope into patches, first into lnore or less trapezoidal shapes and later becoming trianeular planes. Triangular facets on dipslopes appear to be conditioned by very steep inclination and the presence of ridgeforming beds bordering to r,veak layers.
Fig. 1. Diagram of triangular facets dcveloped on steep dipslopes conaisting of ridgefor.ming rock adjacent to wcak strata. Such a phenomenon is to be seen on the Tjilutung dipslopes located bctween Tonro and parakankoudans. west Java.
2. Fro'r the literature rvc knorv the Kei islands in the Banda sea to be tectonically unstable. verbeek (1908) rcported on the fornration of Ut island acconrpanicd by carthcluarkcs in 1852. 'l'hcrcfore, when lve first saw a ferv circular de; press'ions on the rcef flat near Kolseer village, Kai Minor group (figure 2), rve tried to interpret the pits as representing diapiric activity. 1'hese dcpressions are about l0'r across surror.rnded by a low, fractured ri'r of sor'e decirneters height and 2 - 3 nr wide, with inward facing slopes steeper than those sloping outu,ard. Fractures in the rirn ari oriented radially as u'ell as concentrically rvith rcspect to the depressions. The deepest part of the pit in the center is about I rrr deep.
Also i'nfiuenccd by the occurrence of 'rr.rdvolcanoes elservhcre in the islands, the rvriter favoured a natural explosi'e origin for the pits. Luckily, the Kolseer villagers saved the author frorn ernbarrassment and stated that the depressions are bomb craters left by an air raid on the neiehbouring airstrip cltrring the Pacific !Var.
Fig. 2. A bomb crater in the reel flat near Kolseer village, Kai Minor islands. Note concentric and radial fractures in the limestone.'Ihe diameter of the pit is l0 m.
3. F a b r i c analyscs of grar-els genelally statc that loqg pcbble-axes are oriented paralled to the dircction of transport. Pottcr and Pcttijohn (1963, p. 36) citcd exarrrrplcs of pcbble orientations perpendicular to current direction. llods and ellipsoicls in bcach gral'cl arc bclin'ed to lic parallel to thc strandline. In streanrs botir type,s of orientations havc bcen observed. The orientation perpendicular to transportation direction has been cxplained in terms of larges size, ellipsoidal shape, and rolling, u'hereas the smaller particles rio not shorv so distinct an orientation on account of filling-in open spaces left among the larger' pebblcs. Rod-like particlcs are saicl to bc nrore likely oriented perpendicular to the clirection of tra'nspolt, e\cept on foresets u'here these shapes tend to parallel the transportation direction. Potter and Pettijohn (1963) concluded hat pebble orientation seenrs to be governed by shape, size, and probably also by packing density, solting, and streanr gradient.
The rvriter has noted elongated pebbles in stream qravels of Java to be predominantly oriented perpendictrlar to the stream florvs. These pebbles are also inbricated *-ith inclirrations upstream.
Rod-shaped stern frag:nents of ACROPORA like corals on the sandy beaches of Dobo, Aru archipelago, and 'folehu, Arnbon, have preferred orientations of their long axes perpendicular to the strandline. The coral fragrnents vary from a ferv to 10 centimeter lengths and possess diameters of 1 cm or less. Where elongated pebbles of denser rock, like granitic pebbles on Tolehu beach, occur together with the ACROPORA stems, the long axes of the heavier particles are mainly oriented parallel to the waterline (60%, see Tjia, 1965, p. 57). The remainder is oriented diagonally (almost 26%) or perpendicular (13%) to the strandline. It was also noted that the two last mentioned orientations mostly consist of smaller pebbles than those lying parallel to the beach. Figure 3 is a tracing of a photograph showing fragment orientations on Tolehu beach.
Fig. 3. Strips of aligned coral stem fragments (laths) separating broader areas where on particular orientation is discernible. The strips of aligned coral stems may indicate paths of small rip currents (R). Also note the orientation of long axes of granitic pebbles parallel to the strandline. The lower side of the figure is the seaward side.
The long dimension of the figure represents about 1.5 m.
From the writer's observations the following inferences are drawn concerning gravel fabric.
- a. Elongated shape.
- b. Size. Larges and heavier objects are apparently stable when their longitudinal axes lie perpendicular to the direction of movement, whereas lighter particles attain stability through parallel orientation with the current.
- c. Ratio of current velocity and pebble density. A low ratio seems to favour parallel alignments of long pebble axes and current direction.
- d. Packing density. The closer the packing the more abundant are orientations departing from the predominant orientation of long pebble-axes.
+. Sensation of srnoothness rvhich oneobtains when stroking a fault surface parallel to its slickensides, should indicate the direction of movement of the adjoining fault face. All textbooks advocate this method to detect the sence of fault displacements. Recent laboratory and field experience conclusively shou' that most fault-plane features actually are oriented rvith their stceper, and therefore, coarsersides facing into the direction rvhence the adjacent fatrlt plane came and yielding a sensation of coarseness n'hen stroked in the sense as stated above. The protuberances on fault surfaces comprise secondary fractures, gouge marks, and results ot stick-slip and spalling (Paterson, 1958; Tjia, 1964; Rieoker, 1965; Tjia, 1967). Fault-plane fea,turcs u'hich give the sensation of smoothness as expectecl by the classical method do also exist and represent the effects of plucking and smoothing of irregularities throtrgh masking rvith finegrained mylonitc. Interpretation of sense of fault rnovement should, therefore, be based on a study of individual fault plane rnarkings rather than relying on touch.
In Indonesia the author observed fault plane features, which render sensations of coarseness in the sense of fault movement, from faults within pre Tertiary and Tertiary rocks of the Lokulo areal Central Java (1966), a young fault plane of presumably less than 50 years age in Neogene limestone near Dobo, Aru islands (in press), and on rnany slurnp faces in Neogene as well as younger argillites in Java.