Evaluating the role of topographic inversion in the formation of the Stanislaus Table Mountains in the Sierra Nevada (California, USA)

The Table Mountains, a flat-topped series of ridges capped by a 10.4 Ma latite flow in the Stanislaus River watershed, are considered to be evidence for late Cenozoic uplift-driven landscape rejuvenation in the northern Sierra Nevada range (California, USA). The commonly accepted theory for the formation of these mesas posits that the latite flowed and cooled within a bedrock paleovalley and, since then, the surrounding landscape has eroded away, leaving behind the volcanic deposit as a ridge. Although this theory is accepted by many, it has not been thoroughly tested. In this study, I examine a series of geological cross-sections extracted along the length of the latite deposit to determine whether the evidence supports the existence of bedrock valley walls on both sides of the 10.4 Ma flow. I find that the presence of older Cenozoic deposits adjacent to the latite flow precludes the possibility that the flow would have been constrained within a bedrock valley. Moreover, the cross-section from an 1865 report that has been offered as evidence of topographic inversion (and subsequently reproduced in numerous publications) does not accurately represent the topography at that site. I conclude that there is no evidence that the bedrock topography has been inverted and that instead, the latite flowed within a channel cut into underlying Cenozoic deposits, which have since mostly eroded away. This study, therefore, refutes the hypothesis that the Stanislaus River watershed was rejuvenated in the late Cenozoic and challenges the claim for recent significant uplift of the region.

Learning from nature: favoring small lahars formation for hazard mitigation

<p>Lahars represent one of the world destructive natural phenomena as number of casualties (Manville et al., 2013). Lahars originate as mixtures of water and volcanic deposits frequently by heavy rainfalls; they are erosive floods capable of increase in volume along its path to more than 10 times their initial size, moving up to 100 km/h in steeply sloping as far as an extreme distance of hundreds of kilometers.</p><p>Beside tools of early warning, security measures have been adopted in volcanic territory, by constructing retaining dams and embankments in key positions for containing and deviating possible lahars (Leung et al., 2003). This solution could involve a strong environmental impact both for the works and the continuous accumulation of volcanic deposits, such that equilibrium conditions could lack far, triggering more disastrous events.</p><p>The growing frequency of lahars in the Vascún Valley area, Tungurahua Volcano Ecuador, maybe for the climatic change, has recently produced smaller (shorter accumulation periods) and therefore less dangerous events.</p><p>Momentary ponds form along rivers in volcanic areas, when they become usually blocked by landslides of volcanic deposits, which are originated by pyroclastic flows and lahars. The most frequent cause of a breakout of such natural ponds is the overflow of water across the newly formed dam and subsequent erosion and rapid downcutting into the loose rock debris.</p><p>Dam collapse can occur by sliding of the volcanic deposit or by its overturning. By eroding the blockage and flowing out river channel downstream, the initial surge of water will incorporate a dangerous volume of sediments. This produces lahars with possible devastating effects for settlements in their path (Leung et al., 2003).</p><p>The use of simulation tools (from the cellular automata model LLUNPIY) and field data (including necessary subsoil survey) permit to individuate points, where dams by backfills, easy to collapse, can produce momentary ponds.</p><p>Small temporary dams with similar (but controlled) behavior of above mentioned dams can be designed and built at low cost by local backfills in order to allow the outflow of streams produced by regular rainfall events. This result is achieved by properly dimensioning a discharge channel at the dam base (Lupiano et al., 2020).</p><p>So small lahars can be triggered for minor rainfall events, lahar detachments can be anticipated for major events, avoiding simultaneous confluence with other lahars (Lupiano et al., 2020).</p><p><strong>REFERENCES</strong></p><p>Leung, MF, Santos, JR, Haimes, YY (2003). Risk modeling, assessment, and management of lahar flow threat. Risk Analysis, 23(6), 1323-1335.</p><p>Lupiano, V., Chidichimo, F., Machado, G., Catelan, P., Molina, L., Calidonna, C.R., Straface, S., Crisci, G. M., And Di Gregorio, S. (2020) - From examination of natural events to a proposal for risk mitigation of lahars by a cellular-automata methodology: a case study for Vascún valley, Ecuador. Nat. Hazards Earth Syst. Sci., 20, 1–20, 2020.</p><p>Manville, V., Major, J.J. and Fagents, S.A. (2013). Modeling lahar behavior and hazards. in Fagents, SA, Gregg, TKP, and Lopes, RMC (eds.) Modeling Volcanic Processes: The Physics and Mathematics of Volcanism. Cambridge: Cambridge University Press, pp. 300–330.</p>

THE CHARACTERISTIC OF COASTAL SUBSURFACE QUARTENARY SEDIMENT BASED ON GROUND PROBING RADAR (GPR) INTERPRETATION AND CORE DRILLING RESULT OF ANYER COAST, BANTEN PROVINCE

The study of characteristic of subsurfase Quatenary sediment of Anyer coast have been done by using the data of Ground Probing Radar (GPR) image, Surfacial Geological map around the coast and the result of core drilling. The GPR equipment which was used are GSSI SIR 20 system and GSSI Sub Echo 40 MHz antennas. The GPR data image have been processed by using Radan GSSI software, Window NTIM version. The processing including Stacking, Spatial Filter, Migration and Decompolution. The interpretation of GPR image was done by using the principle of GPR stratigraphy through recoqnize to the internal and external reflector such as reflector configuration, continoutas, reflection, amplitude, etc, Furthermore the interpretation result of GPR image are correlated with the surfacial geological map and core drilling result that have been done by previous researscher. Besed on that correlation result, the characteristic of subsurface Quatenary deposits of study area can be divided into 5 unit mainly unit A, B, C, D and E. Unit A is the uppermost layer which is charactized by clay layer and coral reff fragments. Below the unit A they are unit B, C, and D wich were characterized by intercalation between sand and clay, sand deposit or sandstone, loose to dense. This condition is shown by the SPT (Standard Penetration Test) which have range between 10 to 50 blows per 15 Cm. Based on the characteristic of GPR image and sediment deposits of core drilling, these sediment deposits are interpreted as coastal and shallow water sediment deposits. Unit E is the lowermost layer which is interpreted as volcanic deposit. Keywords: subsurface quatenary sediment, ground propbing radar, core drilling, Anyer coast. Penelitian karakteristik sedimen bawah permukaan Kuarter di kawasan pantai Anyer telah dilakukan dengan mempergunakan data citra “ Ground Probing Radar”, geologi permukaan di sekitar kawasan pantai dan data hasil pemboran inti. Peralatan GPR yang dipergunakan adalah sistim SIR 20 GSSI dan antenna MLF 3200 GSSI.Data citra GPR telah diproses dengan mempergunakan perangkat lunak RADAN GSSI versi window NTIM. Pemrosesan terdiri dari “Stacking”, “Spatial Filter”, “Migration” dan “Decompolution”. Penafsiran Citra GPR dilakukan dengan mempergunakan prinsip Stratigrafi GPR melalui pengamatan terhadap internal dan eksternal reflector seperti konfigurasi reflector, kontinuitas, refleksi, amplitude dan lain-lain. Selanjutnya hasil penefsiran citra GPR ini dikorelasikan dengan peta geologi permukaan dan hasil pemboran inti yang telah dilakukan oleh peneliti terdahulu. Berdasarkan hasil korelasi tersebut karakteristik endapan Kuarter bawah permukaan daerah penelitian dapat dibagi menjadi 5 unit yaitu Unit A, B, C, D dan E. Unit A merupakan unit paling atas yang dicirikan lapisan lempung dan kerakal kerikil hasil rombakan koral. Unit B, C dan D berada di bawah ubit A yang merupakan endapan selang seling pasir dan lempung serta endapan pasir atau batu pasir bersifat urai sampai padat. Kondisi ini ditunjukan oleh hasil pengujian SPT(“Standard Penetration Test”) yang berkisar antara 10 sampai lebih dari 50 tumbukan per 15 Cm.Berdasarkan karakteristik fasies citra GPR dan endapan sedimen dari hasil pemboran inti, endapan sedimen tersebut ditafsirkan sebagai endapan pantai dan endapan laut dangkal Unit E merupakan lapisan paling bawah yang ditafsirkan sebagai endapan gunung api. Kata kunci: Sedimen Kuarter bawah permukaan,”Ground Probing Radar”, pemboran inti, pantai Anyer

GEORADAR INVESTIGATION AT THE KEDULAN TEMPLE EXCAVATION SITE, KALASAN, YOGYAKARTA

Kedulan Site is the buried and ruined 9th century Mataram Hindu Kingdom temple, located in Tirtomartani Village, Kalasan District, Sleman Regency, Yogyakarta Special Province. This temple was incidentally discovered by sand diggers on 24 November 1993 under several meter thick of fluvio-volcanic deposit of the modern Merapi. Several technical studies were needed to carefully excavate the temple, including geology and geophysical approaches. One of the geophysical method have been applied was ground penetration radar (georadar). This method uses radar technology to obtain a continuous profile of the shallow sub-surface and thus allows scientists to image soil substratums based on differing dielectric constants. Georadar investigation by Department of Geological Engineering, Faculty of Engineering, Universitas Gadjah Mada, was conducted on 4 December 2007. The main purpose was to identify the location of the outer stone fence as an estimation to define the temple site area to be excavated. About one line was chosen to cross the site in north-south direction in a distance of 328 m. Two runs were completed on the same line but different courses, i.e. forward and backward, where one was checked with another. The result indicates the presence of the outer stone fence was possibly buried in a depth of 7 m. It was located about 40 m distance outside the inner stone fence. Assuming the fences were quadrangle relative to the main temple, hence it is estimated that the site area to be excavated is about 13.830 m² and total 96.808 m³ gravels and sands to be removed.