Field Guide APG 9June organised by TFM

1. Regional Geological Setting

The Subathu–Dagshai–Kasauli–Lower Siwalik succession records the tectono‑sedimentary evolution from marine foreland basin inception to continental molasse development during India‑Asia collision. Marked stratigraphic shifts—tectonic hiatus, paleosols, changing provenance—coincide with structural growth of the Himalayan thrust belt, particularly along the MBF. Field-traced thicknesses, fossils, and structural fabrics provide compelling evidence for this evolving landscape and its ongoing geodynamic significance.

The Kalka–Solan corridor (Parwanoo–Kasauli ridge) lies within the foreland basin, formed during India–Asia collision. Sedimentary units range from latest Paleocene to Pliocene, spanning marine (Subathu) to continental (Siwalik) environments. Structurally, these are bounded south by the Himalayan Frontal Thrust (HFT) and north by the Main Boundary Fault/Thrust (MBF/MBT).

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2. Stratigraphy & Lithology
Subathu Formation (latest Paleocene–mid Eocene 60 to 44million years) in multiple NW–SE bands
Lithology: Olive-green Splintery shales, organic-rich limestones, minor sandstone.
Facies: Shallow marine–shelf; storm beds; abundant Skolithos, Ophiomorpha, Thalassinoides trace fossils

Passage Beds and White Sandstone 43 to 29 million years

Dagshai Formation (Oligocene, 28-23 million years)
Lithology: Red mudstone, siltstone, grey sandstone.
Features: Paleosol horizons (calcrete, root traces), hummocky beds, brackish/floodplain facies

Kasauli Formation (Lower Miocene 23-18 million years)
Thickness: Hundreds of meters along the Kasauli ridge
Lithology: Grey-green micaceous fluvial sandstones, shale lenses, plant fossils.
Environment: Braided megafan/fluvial systems

Lower Siwalik Group (Middle Miocene–Pliocene 18-10 million years)

Lithology: Coarse sandstones, conglomerates, siltstones, thick succession
Facies: Floodplain, meandering to braided river deposits. Vertebrate fossils in Lower/Middle Siwalik.

3. Main Boundary Fault/Thrust (MBF/MBT)

Position: Separates Subathu–Dagshai–Kasauli basin fill (hanging wall) from Lesser Himalayan crystalline units (Simla, Krol, Blaini groups)
Structure: Low-angle thrust with fault breccia, slickensides, intense crushing observed in the field
Activity: Part of thin-skinned fold–thrust system transporting foreland strata northward

4. Structural & Tectonic Framework
Thrust Belt Architecture
Coeval - Surajpur, Bilaspur, Nalagarh/Chakki Ka Mor thrusts cut through Palaeogene strata (Subathu–Kasauli), stacking units and repeating facies

Folds: Multiple NW–SE to NNW–SSE trending synclines and anticlines (e.g. Garkhal, Sanawar, Anji, Dagshai, Barog), intensifying toward the MBT