Sediment Deposits Texture Analysis of Besuk Kobokan River in the Northern Slope Semeru Volcano Lumajang

The flow of volcanic material from Semeru Volcano as one of the most active volcano on Java Island is still flowing and deposited along the Besuk Kobokan River which empties into Bambang Beach, Lumajang. The sediments as volcanic products are transported through the river environment with water as its transport agent. This transportation and deposition process has certain grain texture characteristics that can be identified through granulometry analysis. Geological observations and sediment sampling were carried out at 8 points along the Besuk Kobokan River. Granulometry analysis aims to identify the grain texture distribution of Semeru Volcano sediments that have been transported. The results of granulometric analysis that can identify the distribution of grain texture show that the first point to the fifth point at the sampling location was influenced by lahars flow and pyroclastic flow, resulting in randomness in grain size and rock texture. While the sixth to eighth points show the fluvial sedimentation process.


Introduction
Indonesia with the meeting point of three world plates and the Ring of Fire has many volcanoes scattered throughout the country.One of the active volcanoes on the Java Island is Semeru Volcano.Semeru Volcano, the highest mountain in Java, has an altitude of 3,676 meters above sea level and is included in the Bromo-Tengger-Semeru complex.This volcano is known for producing andesitic magma, which forms cone-shaped volcanic bodies referred to as stratovolcanoes [1].Bogie and Mackenzie (1998) stated that stratovolcanoes are characterized by their steep slopes and the intersection of lava, pyroclastic, and volcaniclastic deposits [2].This mountain administratively located in the Malang and Lumajang is actively experiencing magmatic and phreatomagmatic eruptions [3].Based on satellite image observations, the flow of volcanic material from Semeru Volcano flows and is deposited along the Besuk Kobokan River which empties into Bambang Beach, Lumajang.The volcanic material is in the form of sand, tuff, or ash, and rocks that have a certain flow direction pattern.However, research related to the distribution of grain texture of Semeru Volcano sediments as its volcanic materials has never been conducted.Therefore, this study aims to identify the grain texture distribution of Semeru Volcano sediments that are transported through the Besuk Kobokan River using granulometry analysis.
East Java's regional physiography is grouped into seven zones, namely the Southern Mountain Zone, Solo Zone, Kendeng Zone, Randublatung Zone, Rembang Zone, Alluvial Plain of Northen Java, and Quaternary Volcanoes [4].In the physiographic illustration of East Java by Smyth (2005), Semeru Volcano is included in the Quaternary Volcanoes as a present-day volcanic arc that has been active since the late Miocene [5].Zaennudin (2010) stated that Semeru Volcano generally produces pyroclastic fall sdeposits and pyroclastic flow of collapsed lava flows.Because of its usual volcanic activity, Semeru is classified as effective lahar producers volcano that actively produces lahars [6].
Lahars are a natural phenomenon and are not categorized as deposits.Lahar itself can come from some volcanic eruption material and can also be avalanche material or pyroclastic flows that turn into water flow.The speed, discharge, and transportation stre ngth of lahars are usually higher than river flows.The material content carried by lahars is generally a thick mixture of sand and boulders.Lava flow will interfere with the sedimentation process because the material content is very different between the two [6].

Method
The methodology used in this study is geological observation as field observation and granulometry analysis.Field observations were conducted to identify the field conditions and topography of the sampling location.Then proceed with processing and analysis of granulometric data with 4 parameters: mean, sorting, skewness, and kurtosis to obtain the grain texture distribution from upstream to downstream and deposition patterns in the research area.

Location
The granulometry analysis research area is in the Besuk Kobokan River flow in Pasirian, Lumajang, East Java.This river will empty into Bambang Beach.Samples were obtain ed on October 3 rd , 2022 at 8 points on the point bar side of the river shown in Figure 1 with coordinates shown in Table 1.In point bar side that shown in Figure 2, the sediment has deposited and is not disturbed by river flow.

Material and Method
The samples obtained during field observations are loose materials that have not been cemented.The granulometry analysis is carried out with three main steps, namely sample preparation, data processing, and granulometric data analysis.Sample preparation is carried out to obtain mass data in each mesh size: 4, 8, 16, 30, 50, 70, 140, and 200.Each sample is set to a mass of 500 grams at each point.Then sample is processed to remove the moisture content by heating the sample in an oven with a temperature of 100°C for 24 hours.The dried sample will be sifted and weighed on each mesh.The sample mass data in each mesh will be used at the data processing and granulometry analysis st eps.
Granulometry data processing is carried out through graphical and mathematical methods with a cumulative frequency curve statistical approach to obtain more accurate results.The granulometric analysis parameters used to show the grain texture distribution of the samples are mean, sorting, skewness, and kurtosis.The value of phi(ɸ) is required to get the classification of each parameter.Then, each parameter will be correlated with the distance between points to determine the distribution and change in texture from upstream to downstream in the research area.
Sorting is classified into 7 classes based on the sorting values in Table 3.
Kurtosis is classified into 6 classes based on the kurtosis values in Table 5 below.

Result and Discussion
Based on granulometry analysis on 8 sample points along the Besuk Kobokan River, the values and classifications of mean, sorting, skewness, and kurtosis parameters are shown in Table 6.The mean represents the average grain size spread at each sampling point, the state of sediment supply, and the amount of transport energy.This mean value can determine the energy state and depositional environment of research area [8].The calculation of the mean parameter in Table 7 shows that the variation in grain size in all samples is medium sand and coarse sand.At the first point, the mean value shows 0.562 as coarse sand in the Table 2 classification.Mean value of first to fifth sample point shows the irregular grain size that affected by lahars flow and pyroclastic flow where these flow are dominated by mass flows.Then mean value of sixth to eighth sample point is starting to show grain size characteristic pattern towards the distance as fluvial process in this research area.The mean value and grain size trend were decrease from coarse sand to medium sand.The decrease can be affected by how far the material is transported from the source as shown in Figure 3.  Sortation is determined from the standard deviation value of granulometric analysis calculation.Sortation or particle size uniformity can show the size distribution of the material [9].Standard deviation indicates the degree of sediment sorting.Sorting can be used to identify energy and deposition environments where well-sorted sediments will be deposited in high-energy depositional environments [10] .The calculation results in Table 9 show that each sample has a very good degree.The degree of sorting shows that each sampling point in the Besuk Kobokan River has a high transportation energy so  The skewness value is classified by the symmetry of grain size distribution that can indicate dominant sediments and depositional environment [11].The result of skewness calculations in each sample shown in Table 9 found that the skewness value of the research area is nearly symmetrical to fine skewed.Maity and Maiti (2016) in Atat et al. (2021) said that skewed values that are close to negative or tend to be small are associated with high energy environment [12] [10].It is shown by fifth to eight samples in Figure 5 that the skewness value is decreasing and related with the decreasing current strength from the water as fluvial medium.Kurtosis is calculated as a grain size parameter and related to grain sorting that can identify highenergy environment [13].A curve with a sharp peak (leptokurtic) indicates better sorting in the middle of the distribution value, while a flat curve (platycurtic) indicates poor sorting in the distribution.Based on the results of the kurtosis classification in Table 10, all sample points have a very platycurtic class with value trend in Figure 6 [7].

Conclusion
Besuk Kobokan River flows from upstream on Semeru Volcano and empties into Bambang Beach as downstream.Based on granulometry analysis results that can identify the distribution of grain texture shows that the mean value as distribution of grain size has range from 0.212 -1.648, sorting value 0.105 -179, skewness value -0.0155 -0.1668, and kurtosis value 0.0178 -0.0527.This result shows that first to fifth sample point has irregular texture that has affected by lahars flow and pyroclastic flow.Besides, sixth to eighth sample point shows the fluvial flow characteristics towards the distance.Despite being downstream and in the coastal area, the grain size, sorting level and texture analysis do not reflect a good trend in showing the transportation energy and deposition environment.This is because the distance between upstream and downstream is not far adrift, so the transported materia l is still large.

Figure 1 .
Figure 1.Sampling Points in the Research Area

Table 1 .
Sampling Location Coordinates

Table 2 .
Mean is the average value of the sample in all grain sizes.This average value is based on the calculation of the value of phi(ɸ) that is obtained from the graph between the cumulative curve and the value of phi(ɸ).The mean value can be obtained mathematically by following this equation below.Grain size is classified into five classes inTable 2 based on the mean value obtained from the calculations.Grain Size Classification based on the Mean Value 2.2.2.Sorting.The sorting value indicates the degree of uniformity of grain sorting.Mathematically, the degree of sorting is described by standard deviation.Based on the value of phi(ɸ), the calculation of sorting or standard deviation is below.

Table 3 .
Sorting Classification Skewness relates to the calculation of symmetry from the grain size distribution by the mean.A skewness value close to zero indicates a very symmetrical distribution dominated by fine grains.If the skewness value is negative, then the grain distribution is dominated by coarse grains.The calculation to determine the value of skewness is as follows.
Skewness is classified into 5 classes based on the skewness values inTable 4.

Table 4 .
[7]wness Classification Kurtosis calculation refers to the peak of the distribution curve.If the curve of the distribution results in a flatter one than the normal one, it is called platykurtic.If the resulting curve has many peaks, it is called leptokurtic.Mathematically, the calculation of kurtosis is as follows[7].

Table 7 .
Mean Classification Results

Table 8 .
sediments can be sorted properly.The sorting parameters graph in Figure4shows that the first to fifth sample point has fluctuating curve.Sorting Classification Results that

Table 9 .
Skewness Classification Results

Table 10 .
Kurtosis Classification Results