Determination of the Rock Contact Lithology of the Nanggulan Formation with the Magnetic Method in the Hargorejo Area of Kulon Progo

The Nanggulan Formation is a formation unit consisting of sandstone, clay, and other sedimentary rocks. Lithological contact between clay and geologically old andesite could indicate an avalanche. The magnetic method in collecting data uses a natural magnetic source with a proton magnetometer. Acquisition data was determined according to the survey design, namely a grid with a distance of 250 meters per point. Data processing to remove noise data, namely noise from magnetic storms, position and altitude, and other natural disturbances. Later, Processing will produce a magnetic anomaly map, which must be separated between local and regional anomalies. This local anomaly will later form the basis for 2.5-dimensional modeling. This 2.5-dimensional modeling can get an overview of lithology, relationships between lithologies, and rock contacts according to the geological model in the field. The modeling results can describe the relationship between the claystone of the Nanggulan Formation and the igneous rocks of the Old Andesite Formation. Its relationship claystone is overlapping Andesite rocks in the Horgorejo Kulon Progo area. This can cause soil movement and landslides if the Andesite rocks experience massive weathering and erosion and if rainfall is high.


Introduction 1.Geological of Study Area
The existence of the Nanggulan Formation, the oldest formation, plays a vital role in understanding the formation of basins and stratigraphy in the Kulon Progo Mountains.According to Rahardjo et al., 1995, the distribution of this formation is spread in locations of the Nanggulan-type area, exposed both in the Kalisonggo and Watupuru rivers.Locally, this formation is exposed in the Sermo, Gandul, and Kokap areas [1] (Figure 1).Tectonically, the Nanggulan formation appears due to lifting.The emergence of the Nanggulan formation is influenced by the main horizontal compression force trending southeast.The southeast-trending force produces pairs of dextral and sinistral faults trending northwestsoutheast.At the ends of the sinistral and dextra horizontal faults, folds and reverse faults are formed [2].The Paleogene Nanggulan Formation is in the South-Central Java Basin.In the Kulon Progo Mountains, the main compressional force trending southeast-northwest produces a slope of the sedimentary rock layers (with SE and NW directions), shear joints (E-W and NNWSSE), shear joints (NW-SE), sinistral faults (NNE-SSW), faults dextral (NW-SE), ascending faults (NE-SW), descending faults (NW-SE) and folds (with the NE-SW axis) [3].
The distribution of the Nanggulan Formation on the east side of the Kulon Progo Mountains is very limited.Around G. Mujil, the Nanggulan Formation is found under the Old Andesite Formation.The Nanggulan Formation and Old Andesites can be stratigraphic contacts or faults [4].The Nanggulan Formation biostratigraphy is divided into five zonings.The Nanggulan Formation was deposited at the age of 41.1 million years to 32.2 million years (Middle Eocene to Early Oligocene).According to the horizontal distribution of lithofacies, the deposition of the Nanggulan Formation is influenced by sealevel fluctuations.The reduction in oligotrophic taxa (Sphenolithus) and an increase in eutrophic taxa (Reticulofenestra), especially the small Reticulofenestra (Reticulofenestra spp.), indicates a change in environmental conditions from oligotrophic at 41.1 million years old to eutrophic at 40.40 million years old.The eutrophic increase on the Watupuru and Jetis routes was caused by the addition of eroded land material at 40.40 million years and after 40.40 million years, thereby increasing the availability of nutrients at sea level.This interpretation is supported by an increase in the sediment rate when the sea level decreased slightly at the age of 40.40 million years [5].
The Old Andesite Formation consists of andesitic lava rocks and porphyry andesite.The geological structure that develops is a shear fault.The Old Andesite Formation is exposed in the central, north, west, and southwest of the Kulon Progo elevation.It was deposited in a volcanic environment composed of volcanic breccia, lava, lapilli breccia, lapilli tuff, and volcanic sandstone.The dominant constituent lithology consists of andesite breccia with a matrix of sandy tuff, fragments consisting of pyroxene andesite to hornblende andesite.In addition, igneous intrusions are also revealed [6].

Magnetic
The basic understanding of the geomagnetic method is the basic theory of the geomagnet itself.The theories discussed include Magnetic force, magnetic field strength, magnetic susceptibility, and rock classification based on the rock's magnetic value.
If there are two magnetic poles and a distance of r, then there will be a magnetic force equal to: where ߤ is the permeability of the medium in a vacuum, dimensionless, and has a value of one, while denotes the unit vector with the direction to Telford et al. [7].The magnetic field strength H is the force on a magnetic pole (2) when placed at a point in a magnetic field resulting from the pole strength.
where r is the distance of the measurement point.It is assumed that it is much larger than, so it does not cause interference with the H field at the measurement point.The unit of a magnetic field in SI is ampere/meter (A/m), while in cgs, it is oersted, where oersted is 1 dyne/magnetic pole [8].
Magnetic susceptibility can also be interpreted as the degree of magnetism of an object.The value of k in the rock will be greater if there are more magnetic minerals [9].Based on the magnetic susceptibility value, materials are divided into ferromagnets, paramagnets, and diamagnets.Data processing is reduced to the poles and separated between regional and local anomalies.Reduction to the poles is the processing of magnetic field anomalies by changing the direction of the earth's main magnetic field (at + -) as if it were at an inclination of 90° or the poles.This process eliminates the dipole effect due to the inclination and declination effect so that the anomalous object can be right under the anomalous curve.In addition, to reduces ambiguity when interpreting anomaly profiles and facilitates interpretation when correlated with other geophysical anomalies such as gravity.The reduction to the poles is carried out using the Oasis Montaj ver 8.4 software through the MAGMAP menu and selecting the Reduce to Magnetic Pole filter.
The total anomaly of RTP (Reduction to Pole) results consists of regional anomaly, residual anomaly, and noise.To separate the anomaly, a Bandpass filter is used.The anomaly separation process uses Oasis Montaj through the MAGMAP menu and selecting a bandpass filter.The difference in the anomaly area can be seen using the spectrum curve, namely through the difference in the slope of the curve graph.

Method
This study used a literature study on the Hargorejo, Nanggulan, Kulon Progo and surrounding areas.Perform magnetic data retrieval.This magnetic data retrieval uses the Proton Magnetometer processor.Apart from these tools, a geological compass, GPS, and a clock are also used while in the field.Retrieval of geomagnetic data will provide variations vertically and laterally.Rock contrast can be clearly distinguished based on the value of rock susceptibility.
Geomagnetic studies in this research field will take measurement points based on the grid method in a region.The grid's points are 250 meters apart (Figure 2).The findings of the magnetic data collection will determine the distribution of susceptibility values for various rocks in the research area.Studio analysis in the form of processing and processing field data in geo-magnet using Mag 2 DC and Surfer software.The processing results will get a two-dimensional cross-section interpreted using classification [10].The results of geomagnetic data processing will provide a vertical and lateral picture of the subsurface in the form of distribution, rock dimensions, lithology conditions, and geological structures, especially rocks that have the potential to become slip planes in cases of ground motion that have occurred.The thickness of rocks with high and low magnetic content will be presented in the Magnetic Anomaly Map.
Field mapping was used to collect data through in-depth observations of rock outcrops in the Nanggulan Formation and Old Andesite Formation.Megascopically, observations of rocks' color, structure, texture, and composition are made.Megascopic naming of sedimentary rocks based on grain size theory [11].The mineral composition in the Bowen series referred to based on the classifications [12], is used to name igneous rocks.This research targets the boundaries between rock units and the distribution of rocks.A geological map will be created as a result of mapping rock outcrops.The boundaries of the rock units in the field will be compared to those of the rock units in the study area's cross-section of the magnetic anomaly map.Based on the cross-section of the magnetic anomaly, it is hoped that the results of determining the contact boundary will provide an overview of rocks' distribution horizontally and vertically.

Results and Discussion
The oldest rock formation in the research area, the Nanggulan Formation as shown in the regional geological map exposed to the surface in several places on the east side of the Kulon Progo Mountains, surrounded by a younger rock formation, the Old Andesite Formation (Figure 3).As expressed in Figure 4, the outcrop verifies the possibility that the claystone Nanggulan Formation was exposed to the surface due to the massive intrusion of the Old Andesite Formation.The intrusion that massively happened pushed upward the claystone to the surface.The small distribution of claystone Nanggulan Formation on the surface might controlled by the morphology and the rock's low resistance.
The rock contact of the Nanggulan and Old Andesite Formation is also supported by Winarti et al. [13] who stated that The Nanggulan Formation is characterized by undisturbed rock layers that are relatively horizontal and were elevated to the surface as a result of andesite dike intrusion.The sedimentary rock layers from the Nanggulan Formation form a roof pendant to the Old Andesite Formation dike above.
Morphologically, the claystone Nanggulan Formation was positioned above the andesite intrusion weathering, erosion, and transportation processes.Due to the rock's low resistance, exogenic processes took intensive impact that might cause the weathered claystone to be transported and deposited in the surrounding lowland.Total magnetic field corrected for daily variations and IGRF will result in a total anomaly magnetic field.The total magnetic field anomaly still has a dipole effect due to the influence of the inclination and declination angles of the study area.In addition, the total magnetic field anomaly also consists of sources of shallow depth anomalies (residual anomalies) and deep depth (regional anomalies) (Figure 5).
The image below results from gridding the total magnetic field anomaly in the study area.The anomalous range is from -325.2 nT to 802.3 nT.The purple color indicates a high magnetic field anomaly while the blue color represents a low anomaly.High anomalies dominate the study area's south, northeast, and center.While low anomaly in the northern, central, and a small part of the west of the study area.
The dipole effect impacts the total magnetic field anomaly map where the anomaly curve is not directly above the source.As a result, reduction to the poles (RTP) is required to neutralize the dipole effect.The dipole magnetic field anomaly transforms into a monopole, with data collection occurring at the poles.The anomaly's source is therefore thought to be below the anomaly curve.Inclination and declination measurements for the study area are -31.135°and 0.767°, respectively.Blakely [14] asserts that converting regions with an inclination value of less than 25 to the equator is preferable because doing so would result in an unstable mathematical equation and inaccurate results.
A map of the entire magnetic field anomaly that has been condensed to the poles is shown below.The range of the anomaly intensity value is -903.1 nT to 1230.5 nT.The anomaly pattern and magnetic intensity values vary concerning the total magnetic field anomaly.This shows that the contour of the anomaly is above the source of the anomaly.The study area's southern, southwest, eastern, and central regions have high anomalies.Anomalies range from moderate to low in the north, northwest, parts of the south, and west.
According to Figure 5, the variation in magnetic anomaly values denotes the emergence of faults with a northwest-southeast trend.These faults have the same direction and are paired with dextral faults.A contour offset is considered to be present at high magnetic anomaly values (pink).It is believed that tension brought on by the presence of the two dextral faults gave rise to the downward fault.
A minor descending fault in the field that is trending primarily west-east confirms the existence of the descending fault (Figure 5).According to Widagdo and Permana [15] studies, this is accurate.Magnetic field anomalies from RTP include regional anomalies (deep depth), residual anomalies (shallow depths), and noise.It is necessary to separate the anomaly to ensure the source of the anomaly.In the separation of anomalies, a bandpass filter is used.Regional anomaly is a response from an anomaly source at deep depth or has a low wave number.While the residual anomaly is a response from an anomaly source near the surface or with high wave numbers.
To get regional anomalies, you can apply a lowpass or bandpass filter with a cut off which is limited to low wavenumber values up to the limits of regional and residual anomalies (differences in the slope of the spectrum graph).Meanwhile, to get the residual anomaly using a bandpass filter with a cutoff slope difference between regional and residual anomaly and residual anomaly with noise (spectrum image) Figure 6.
Figure 6.Spectrum curve, the red line of the regional area, the blue line of the residual area, the green line of the noise region.
The B-B' cross section (Figure 6) passes through the Andesite Intrusion, Nanggulan, Kebobutak, and faults.This incision is trending east-west with a length of 3,160 m.The formations that compose under the surface of the B-B' cross section successively from old to young are the Nanggulan Formation with a susceptibility value of 0.009-0.032SI, the Kebobutak Formation with a susceptibility value of 0.025-0.048SI, and Andesite intrusion with a susceptibility value of 0.016-0.104SI.
The fault shown in the model shows a downward fault mechanism with the left block relatively down from the right block.The Nanggulan Formation appears to be hanging, possibly due to the intrusion process, which then raised the Nanggulan Formation below the top of the intrusion.The top column is a graph of the anomaly curve, the black point of the data curve, the black line of the model curve, the red line of the error curve.The lower column is a cross-section of the subsurface model in the form of the Nanggulan Formation (green), the Kebobutak Formation (brown), Andesite intrusion (red), the black dotted line is an indication of a fault, the arrows are the fault.
Field data gravity shows that the contact between the Nanggulan Formation and Old Andesite is in the form of a vertical stratigraphic contact, with wavy formation contact boundaries.The density value of the Nanggulan Formation is 2.5 g/cm 3 , while the Old Andesite Formation is 2.7 g/cm 3 [16].The anomalies that appear in G. Mujil are caused by differences in lithology so that the contact between the Nanggulan Formation and Old Andesites is stratigraphic.
Unconformity areas are often found in the Nanggulan rock layers.One is the layer; the condensed section is an unconformity field in the middle neritic depth because it is called ocean unconformity [17].Based on the petrographic data and the intersecting relationships in the Old Andesite Formation field, it is known that pyroxene andesite rocks are formed first as shallow intrusions, then formed hornblende porphyry andesite as a deeper intrusion and breaks through pyroxene andesite [18].The intrusion boundaries and rock units in the Old Andesite Formation show the presence of typical faults.Additionally, this is present in Kulon Progo [19,20], particularly in the western portion of the Kulon Progo Mountains [21,22].

Conclusion
In conclusion, this study sheds light on the Nanggulan Formation and its relation to potential avalanches in the Horgorejo Kulon Progo area.The magnetic method, aided by a proton magnetometer and natural sources, enabled systematic data collection and noise reduction, resulting in a magnetic anomaly map.The subsequent 2.5-dimensional modeling revealed intricate lithological relationships, emphasizing the overlay of claystone on Andesite rocks.This juxtaposition raises concerns about soil movement and landslides, especially during heavy rainfall and significant weathering.This research

Figure 1 .
Figure 1.Regional geological map of the study area [1].

Figure 2 .
Figure 2. Geomagnetic data collection point in the research area.

Figure 4 .
Figure 4. Rock Contact of Claystone-Nanggulan Formation and Old andesite Formation.

Figure 5 .
Figure 5. Map of anomaly magnetic study area.

Figure 7 .
Figure 7. Compilation of geological map and the B-B' cross section of magnetic sub surface.