Grain-size distribution dataset of lagoonal and riverine coastal placer deposits along the southeastern coast of Sri Lanka

The dataset contains grain size data of placer and non-placer sediments in lagoonal and riverine beaches of southeastern part of Sri Lanka. A total of 124 swash sediment samples were collected from a 70 km long coastline with an interval of 500 m. Placer sediments in the area mainly have mineralogy of ilmenite, zircon and almandine while non-placers are quartz, albite and calcite. After dry sieving, the grain size distribution (GSD) analyses were carried out on each sample using the Gradistat Excel template. Placer deposits result coarse-skewed leptokurtic to platykurtic fine sand distributions while non-placers are medium sand-grained. The dataset can be used to interpret the deposition environment and transportation dynamics. Further, they can be used to study the southwestern coastline of the Bay of Bengal, juvenile crust sediments of Grenvillian age, alongshore and fluvial sediment dynamics, depositional and erosion processes, geohazards assessments and heavy mineral deposits.


Value of the Data
• The data pertains to unexplored heavy mineral placers in the southeastern coast of Sri Lanka.
• The data focuses on the southwestern coast of the Bay of Bengal and examines variations in grain size distribution (GSD) [ 2 ]. • Vijayan Complex, which is the basement and sediment source of study area, is related to the evolution of an island arc, juvenile crust of Grenvillian age, Lützow-Holm Bay region in East Antarctica [ 3 , 4 ].• The data show the GSD variations between the following: ilmenite, garnet placer deposits and siliciclastic sediment; crescent-shaped bays and straight beach sediment; numerous fluvial outlets and their gradual sinking distance increment on beach; and alongshore ( ∼70 km) beach sediments.• The data can be reused for the studies of alongshore and fluvial sediment dynamics, depositional and erosion processes, engineering applications, Precambrian host rock, renewable heavy minerals and Bengal Bay sediment.• The data can be beneficial to heavy mineral industries, sedimentologists, researchers, and civil engineers.

Background
The dataset was designed to distinguish grain size characteristics, sediment transport and depositional processes, and depositional environments of heavy placers from siliciclastic deposits.Also, the dataset can be used to understand the alongshore and fluvial sediment dynamics, nearshore coastal processes, coastal erosion, temporal variations, geohazards assessments and Bengal Bay sediments.This article presents raw data that are not included in the related research article.The raw dataset may be useful to researchers for creating cumulative passing presentation graphs and various bivariate graphs and statical analyses.

Data Description
In Fig. 1   Akkaraipattu beach samples show an increasing trend towards the south direction.Fig. 2 shows the field photographs of the different deposits found in the study area.
The main fluvial outlets of Gal Oya, Karanda Oya, Heda Oya and Wil Oya result in between SE1, SE99-SE100, SE111-SE112 and SE118-SE119, respectively.Mostly, scatter distribution of fine sand placers can be observed around the southern part of these fluvial outlets ( Fig. 1 ).The heavy mineral contents of Komari and Periya Kalapuwa are observed to be higher black in color ( Fig 2B -C ).Whereas, the beach sector in between Heda Oya and Wila Oya shows a high amount of garnets by giving the red color nature to the beach ( Fig. 2D ).The mineralogy of the area's placer deposits consists primarily of ilmenite, zircon, and almandine, while non-placers include quartz, albite, and calcite [ 1 ].
As geomorphological observations, the northward development of spits with granitic gneiss rock boulders resulted along the nearshore areas of Sangamankanda to Panama beach.Further, the southern half of the present study area showed a higher number of coastal dunes.Retention of heavy mineral populations is observed to be on the berm zone or swash zone with a depth of ∼10-15 cm ( Table 1 ).
In Fig. 3 , distributions can be categorized into three modes such as coarse sand, medium sand and fine sand modes.Study samples abundantly show the medium sand populations, while most of the fine sand populations are related to the placers.
The plot shows the sediment population variations from fine, medium to coarse sand through a helical trend.Their bi-variations are projected in the figure in red and blue colours.A similar kind of plot resulted in the study of [ 5 ].As in the mean vs. skewness projection, the studied samples show a zigzag trend that separates four skewed natures.The mean vs. kurtosis projection also shows the clustering of present samples within a sinusoidal trend ( Fig. 4 ).ment samples were collected at an interval of 500 m covering a distance of 68 km, from Oluvil to Panama coast ( Fig. 1 ).During sampling, about 5 kg of sediments were collected from a small pit on the swash zone with an average depth of 15-30 cm in each sampling point.Plastic shovels and zip-lock bags were used to collect the samples.Field photographs and sampling locations were recorded at each point.
Before sieve analysis, the samples were air dried to make the grain free from trace moisture.Then, the dried sediment samples were subjected to coning and quartering to obtain representative samples.Carbonate material and salt coatings were removed by 10 % dilute hydrochloric acid and deionized water [ 6 ].These sediment samples were oven-dried at 60 °C for 24 h [ 6 ].Finally, sieve analyses were carried out by Retsch AS 200-digit model digital shaker at 1 phi intervals using ASTM sieves for 15 min [ 5 ].Grain size distribution and graphical statistical parameters namely, mean, sorting, skewness and kurtosis were estimated using the Gradistat Excel template following the [ 5 ] method.The tri-bivariate scatter plots of the statistical parameters were plotted using Origin Pro (2021) to understand the relationship between different parameters [ 5 ].

Limitations
Due to the study's budget, the dataset could not capture seasonal variation.Due to coastal defence systems, regular sampling intervals at Akkaraipattu and Panama Beach were disrupted .

Fig. 1 .
Fig. 1.Geological map of the study area shows the sampling locations, lagoonal and riverine environments.

Fig. 2 .
Fig. 2. Examples of the sedimentary deposits from which the samples were taken.A) Siliciclastic deposits, B) and C) ilmenite placers and their vertical profile, and D) Garnet placers.The scale of plastic shovel is nearly 15 cm long.

Fig. 3 .
Fig. 3.The plot shows the grain size distributions of samples.

4 .
Experimental Design, Materials and MethodsSediment sampling was carried out from October 2019 to February 2020 during the northeast-monsoon season.The sampling season was selected to relays with the higher fluvial outflow time.During the preliminary field visits, placers were recorded between 2 and 3 km near the coastline from fluvial outlets.Hence, the study area was classified into 10 segments based on the fluvial outlets, beach morphologies and depositions types, namely (i) Akkaraipattu Straight Coastline (18 sample points), (ii) Periya Kalapuwa Deltaic Environment (21 sample points), (iii) Korai Deltaic Environment (11 sample points), (iv) Sangamankanda Point Coastline (14 sample points), (v) Komari Deltaic Environment (11 sample points), (vi) Komari Straight Coastline (7 sample points), (vii) Pottuvil Deltaic Environment (12 sample points), (viii) Arugam Deltaic Environment (11 sample points), (ix) Heda Oya Deltaic Environment (13 sample points) and (x) Wila Oya Deltaic Environment (6 sample points).Sampling was designed to find the characteristics of different deposits and coastal types.As a final measure, a total of 124 sedi-

Fig. 4 .
Fig. 4. The tri-variate scatter plot shows the variation of mean, skewness and kurtosis of the samples.

Table 1
Location data and grain size parameters of the samples in phi scale.The underlined and bolded mean values show the placers of the area.