The reconstruction of 3D geo-lithological model of Pekan, Pahang: A possible onshore extension of Penyu Basin

Abstract

Well studies have confirmed the presence of Quaternary sediments in both onshore and offshore areas near the township of Pekan in Pahang, while the Tertiary sediments are only found offshore. This could be well-explained through the palaeo-environmental evolution of the Tertiary sediments in Penyu Basin, where it started from continental (older sediments) to marine (younger sediments) environments, suggesting that transgression had occurred causing the Pahang River delta to be located where it is today. In fact, the presence of ancient river channels found in the Penyu Basin supports the transgression claim. Thus, to prove the possible extension of the basin, this paper aims to delineate the geometry of the basement at Pekan area, as well as to study the distribution of different basement rocks in the area by reconstructing the geo-lithological model of Pekan. Thirty (30) well data from previous work, conducted by the Jabatan Penyiasatan Kajibumi Malaysia in 1992 and 1993 were used to reconstruct the geo-lithological model. Utilizing the Lithoblending algorithm to reconstruct the model has allowed us to have a greater control on the model by using nearby well data to extend the lithology laterally, rather than extrapolating vertically below the boreholes, which can lead to inaccuracy in correlation. The outcome of the geo-lithological modelling revealed that the basin originates 5km from the present shoreline, deepens and widens towards the east, i.e. offshore area. In addition, two different types of basement rocks were identified in the area, namely weathered granite and metasediment. The contact between these two basement rocks, are found to be underlying the present-day Pahang River. The deepening and widening of the basement as well as the contact point between the basement rocks is interpreted as corresponding to the opening of the Penyu Basin.

On The Supposed Onshore Extension Of The Penyu Basin, Peninsular Malaysia

“…More importantly, the gravity profile indicates that the Tertiary outcrops at Chenor clearly lie in a basin that is separated from the Penyu Basin by a regional basement high composed of a mixture of Palaeozoic metamorphic and Mesozoic plutonic rocks. This is confirmed by the groundwater boreholes in Figure 7 which penetrated pre-Tertiary basement comprising mainly weathered granites north of Pahang River and mainly metamorphic rocks south of it (Yoong et al, 2018). 5 showing the correlation with pre-Tertiary outcrop geology (lower panel).…”

“…This line, which is also above sea level, may be taken as the landward limit of the basin (Figure 4B). Yoong et al (2018) reported groundwater boreholes that were drilled through the Quaternary sediments in the Pekan area (red dots in Figure 3). The boreholes penetrated as much as 115 m of Quaternary sediments before reaching the bedrock of pre-Tertiary geology comprising weathered granites and metasediments.…”

“…Grey shaded area landward of the coastline represents Quaternary deposits on the coastal plain (Raj et al, 2009). Red circles are the locations of groundwater boreholes from Yoong et al (2018). Yellow square is the location of East Chenor outcrops studied by Makeen et al (2019a, b), as mentioned in the text.…”

“…Doust & Sumner, 2007). I suggest that it serves no purpose to "prove" that Penyu Basin extends onshore based on the presence of Quaternary sediment, as attempted by some authors (e.g., Yoong et al, 2018). For practical purposes, where direct structural evidence (e.g., faults) is lacking, other practical means may be used to define the approximate limits of sedimentary basins, such as sediment thickness, basement depth, or gravity anomalies.…”

“…Note the location of the Chenor outcrops (yellow square) within an area mapped as Jurassic-Cretaceous sedimentary rocks (JK). Groundwater borehole locations (large white circles on map) are from Yoong et al (2018).…”