Ekskursi Geologi Regional Lampung

Summary

The transcript provides an in-depth exploration of Sumatra's geological evolution from the Oligocene to the Quaternary. It details the formation of volcanic arcs, large Sumatran faults, and sedimentary basins, highlighting the three main rock lanes: Juru Penghulu, Bukit Barisan, and Palembang. The discussion covers metamorphic, igneous, and sedimentary rocks, including major formations like Lampung, Gading, Talang Akar, Baturaja, and Sukadana basalt, and examines intrusions from the Cretaceous to Miocene. Key processes such as subduction, magma formation, and depositional environments are emphasized, providing insights into the tectonic and volcanic history shaping Sumatra's complex landscape.

Takeaways

• 🌋 The large Sumatra volcanic system became active during the Oligocene, triggered by the subduction of the Indian Ocean plate beneath Sundaland, forming a volcanic arc and the Sumatran Fault.
• 🏞️ Sumatra's sedimentary basins are divided into three main lanes: Juru Penghulu in the west, Bukit Barisan in the middle, and Palembang in the east, shaped by volcanic arcs and tectonic activity.
• 🪨 The Bengkulu lane contains sedimentary rocks deposited in a forearc basin, influenced by flexural bending due to tectonic push.
• ⛰️ Bukit Barisan is a northwest-southeast oriented mountain range controlled by the Sumatran Fault, exhibiting transpression and transtension structures like pop-up and flower structures.
• 🌊 The Palembang lane represents a back-arc basin formed due to the back-arc extension from the subduction process of the Indian Ocean plate.
• 🪨 Lampung region metamorphic rocks form the basement and include schist, quartzite, marble, and migmatite, often associated with igneous intrusions due to high-pressure metamorphism.
• 🪨 Sedimentary formations in Lampung, such as Nyamikan, Lemawong, and Simpang Our, record environments from submarine fans to shallow marine and volcanic activity.
• 🌋 Volcanic intrusions in Lampung include Cretaceous granitoids, Paleogene granite, and Neogene dacite, cutting through metamorphic complexes and indicating multi-phase magmatism.
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• 🏞️ The Talang Akar, Baturaja, Gumai, Kasai, and Sukadana formations reflect depositional environments from fluvial and shallow marine to reef and continental basalt, providing petroleum source and reservoir rocks in South Sumatra.
• 🔥 Sukadana basalt is unique as a continental back-arc basalt, likely formed through partial melting and magma assimilation due to its anomalous location behind the volcanic arc.
• 📍 The geological history of Sumatra integrates tectonics, volcanism, sedimentation, and magmatism, highlighting complex interactions between subduction, faulting, and basin formation over the Cretaceous to Quaternary.

Q & A

What geological event triggered the formation of volcanoes in Sumatra during the Oligocene Barbary period?

The formation of volcanoes in Sumatra was triggered by the large Sumatra system becoming active at the same time as the Indian Ocean plate subducted under Sundaland, which caused magma formation and led to the emergence of a volcanic arc.

What are the three major rock lanes in Sumatra and how are they positioned?

The three major rock lanes in Sumatra are the Juru Penghulu lane in the west, the Bukit Barisan lane in the middle, and the Palembang lane in the east. They are defined based on the position of the volcanic arc and the horizontal influences from the Sumatra fault.

How did the Bengkulu lane form, and what is its geological significance?

The Bengkulu lane formed due to tectonic movements, particularly the bending or flexure of the Earth's plates due to horizontal push forces. This caused the formation of sedimentary rock basins in the region.

What is the primary geological feature of Bukit Barisan and what role does it play in the region's formation?

Bukit Barisan is a mountain range formed by a large fault system. Its formation is controlled by the large Sumatra fault, which causes the range to have a northwest-southeast orientation. It also influences the formation of positive and negative structural features in the surrounding area.

What role did volcanic activity play in the deposition of the Simpang Our formation?

The Simpang Our formation, composed of tuffaceous sandstone and limestone, was deposited during a time of volcanic activity on Sumatra, which left its signature in the formation's lithology.

Can you describe the different types of metamorphic rocks found in Lampung and their significance?

Lampung's metamorphic rocks are highly diverse, including schist, quartzite, and marble. These rocks are crucial as the base rocks for sediment deposition in the region, and they have been classified into various groups such as the Kasih Mountain complex.

What is the significance of the Gading formation in the context of the South Sumatra basin?

The Gading formation is significant because its lower part acts as a petroleum source rock for the South Sumatra basin, while the upper part serves as the reservoir system for the basin.

What are the characteristics of the Sukadana basalt, and why is it considered an anomaly?

Sukadana basalt has a unique scoria structure, which is formed due to gas release during volcanic activity. It is considered an anomaly because it is found behind the volcanic arc, where typical magma would usually undergo differentiation and assimilation with surrounding rocks.

How did the different tectonic settings influence sediment deposition in Sumatra?

The tectonic settings, particularly the flexural bending caused by the Sumatra fault, influenced the deposition of sediments in various basins, resulting in the accumulation of distinct rock formations such as those found in the Bengkulu, Bukit Barisan, and Palembang lanes.

What is the relationship between the Lampung formation and the Miocene volcanic activity in Sumatra?

The Lampung formation is closely tied to the Miocene volcanic activity in Sumatra, which occurred due to the subduction of the Indian Ocean plate. The formation includes volcanic deposits such as tuffaceous sandstone and pyroclastic flows.