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Rajmahal Traps

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Nazim Naeem

A presentation on Rajmahal Traps submitted to Dr. Badrul Imam, Faculty member of Department of Geology, University of Dhaka by Md. Nazim Uddin.

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Presentation on Rajmahal Traps
Do you know how many lava flows are there in Rajmahal Traps?
Introduction It consists of 15 lava flows and 11 intertrappeans. The Rajmahal Stage in the type area consists of 450m to 600m of basaltic lava interbedded with contemporaneous shale and clay beds Total thickness of the intercalated sedimentary beds is only 30m. The top-most flow is the thickest one and is 76m, but others are thinner. The Rajmahal Stage/Traps is named after the Rajmahal Hills, Bihar, India.
Genesis The age of Rajmahal Traps is highly controversial. Microfloral assemblage (provided by four lowermost intertrappeans bands) are indicative of Jurassic age. The upper portions of the stage represent the Lower Cretaceous ( K-Ar determination). So the Rajmahal Stage has a stratigraphic range from Lower Jurassic to Lower Cretaceous.
Subcrops in Bangladesh Subcrops of the stage known from the Bogra Graben, Jamalganj and Sibganj area in the westernmost part of Bangladesh. Marginal remnants of it were encountered in Jamalganj area. The Bogra graben also shows the stage with reduced thickness for erosional events.
Lithology The lava flows consist mainly of hornblende basalt, olivine basalt and andesite. The basalt are commonly dark colored, porphyritic with fine grained matrix. Due to weathering, the basalt fragments have a grey-green color in the upper portions, particularly in the Singra area. The intertrappeans consists of an alternation of variegated siltstones, sandstone and conglomeratic lenses.
Conclusion Age assignment of these beds is still an unsolved problem due to lack of microfloral or faunal record. The Rajmahal stage is assigned to Jurassic/Lower Cretaceous which represent the entire age of this stratigraphic unit in the type area.
Thanks For Being with Md. Nazim Uddin Roll: AE-012 Department of Geology, University of Dhaka

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Rajmahal Traps

  • 2. Do you know how many lava flows are there in Rajmahal Traps?
  • 3. Introduction It consists of 15 lava flows and 11 intertrappeans. The Rajmahal Stage in the type area consists of 450m to 600m of basaltic lava interbedded with contemporaneous shale and clay beds Total thickness of the intercalated sedimentary beds is only 30m. The top-most flow is the thickest one and is 76m, but others are thinner. The Rajmahal Stage/Traps is named after the Rajmahal Hills, Bihar, India.
  • 4. Genesis The age of Rajmahal Traps is highly controversial. Microfloral assemblage (provided by four lowermost intertrappeans bands) are indicative of Jurassic age. The upper portions of the stage represent the Lower Cretaceous ( K-Ar determination). So the Rajmahal Stage has a stratigraphic range from Lower Jurassic to Lower Cretaceous.
  • 5. Subcrops in Bangladesh Subcrops of the stage known from the Bogra Graben, Jamalganj and Sibganj area in the westernmost part of Bangladesh. Marginal remnants of it were encountered in Jamalganj area. The Bogra graben also shows the stage with reduced thickness for erosional events.
  • 6. Lithology The lava flows consist mainly of hornblende basalt, olivine basalt and andesite. The basalt are commonly dark colored, porphyritic with fine grained matrix. Due to weathering, the basalt fragments have a grey-green color in the upper portions, particularly in the Singra area. The intertrappeans consists of an alternation of variegated siltstones, sandstone and conglomeratic lenses.
  • 7. Conclusion Age assignment of these beds is still an unsolved problem due to lack of microfloral or faunal record. The Rajmahal stage is assigned to Jurassic/Lower Cretaceous which represent the entire age of this stratigraphic unit in the type area.
  • 8. Thanks For Being with Md. Nazim Uddin Roll: AE-012 Department of Geology, University of Dhaka