That doesn’t mean that there was no Miocene Ganga

That doesn’t mean that there was no Miocene Ganga, but that the exposed Siwaliks are the wrong place to look for it. An axial river would have developed a little further south of the alluvial fan complexes prograding from the orogeny front and so older axial river channels are likely buried beneath the Gangetic Pleistocene-Holocene alluvium. The oldest sediment deposited by the Ganga which is currently exposed in the Gangetic plains is about 120 thousand years old. But there is 2-3 km of older sediments ranging in age from Miocene to mid Pleistocene underneath this recent alluvium. In effect, the Siwaliks continue in the subsurface of the Gangetic plains. This next map shows the tectonic elements of the Himalayan system and sediment thicknesses in the Gangetic plains.

It shows that the exposed Siwaliks ranges (in map- southern boundary of frontal folded zone) are just the deformed northernmost part of the foreland basin . All those transverse streams eroding the Himalayas and depositing sediment in the proximal part of the Siwaliks would have met an axial river flowing in the distal parts of the foreland, more to the south of the orogenic front.

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There is another geochemical clue that a paleo-Ganga and paleo-Brahmaputra along with a paleo-Indus were draining the Himalayas since the Miocene and that evidence lies in the changing 87Sr (Strontium) isotope and 187Os (Osmium) isotope composition of sea water. The reasoning is that the uplift of the Himalayas exposed enormous volumes of new rock which were rich in these elements. Chemical weathering of these rocks leached these elements into rivers and ultimately into the ocean. Take a look at the plot of changing 87Sr and 187Os isotope of sea water through the Cenozoic. The Sr isotope composition of ancient sea water is estimated by measuring Sr incorporated in CaCO3 shells of marine creatures. The Os isotope composition of sea water is estimated by measuring Os incorporated in manganese hydroxide nodules precipitated in deep sea sediments.
geologists have found out that beginning around 15 million years ago, Mn (manganese) oxide nodules precipitated in the floodplains of river deposits are enriched in 187Os. The weathering and erosion of Krol-Tal strata released clay and silt size particles which got deposited in floodplains. These floodplain sediments were chemically weathered and released 187Os, which due to its affinity to Mn got incorporated in the growing Mn oxide nodules in the soils forming on floodplain sediments. The ground water that was carrying this 187Os also released that element in the river. These transverse rivers would have joined an axial river, the paleo-Ganga to the south, thus carrying the 187Os signal to the Bay of Bengal.
Brahmaputra rive paleo study :
Compilations of sand thickness and sand/shale ratio of the Miocene Surma Group show that Lower to Middle Miocene strata of the Bhuban Formation accumulated in a large, elongate trough. Sand thickness and percentage both decrease markedly away from this depo center, which describes a large-scale bend, running initially westward from Rashidpur (northeast Bengal basin) and curving southward toward the Bengal fan. Middle to Upper Miocene strata of the Boka Bil Formation show a similar geographic trend in deposition of coarsest and thickest sediment, but the major depo center had shifted northward relative to that of the Bhuban Formation by some 30 km, passing near Beani Bazar. These trends suggest that deltaic deposits of the Surma Group filled the Sylhet trough of the northeast Bengal basin from the east. Published seismic data from western Bangladesh show that additional large channels also contributed materials to the Bengal basin from the northwest during the Late Miocene, but these channels resulted in very little accumulation in the northwestern part of the basin, probably due to restricted subsidence of underlying continental crust. This study suggests that there was a major drainage system similar to the modern Brahmaputra River during Miocene time, which carried orogenic sediments eroded from the uplifted terranes of the eastern Himalayas and Indo-Burman ranges to the eastern Bengal delta.
Core sediment sample of BOB:Based on down-core pro?les of sediment grain size, magnetic studies and elemental variations, the core was broadly divided into three units: unit 3 (70–43 cm) represents a clayey-silt organic rich turbidity, unit 2 (43–24 cm) consists of coarse-grained terrigenous sediments whereas biogenic sediments dominate unit 1 (24–0 cm). Based on age (14 CAMS) vs. depth it was observed that units 3 and 2 have higher sedimentation rates (7.8 cm/ka) than unit 1 (5.5 cm/ka). The average sedimentation rate for unit 3 was calculated for 60–43 cm interval as 14C AMS dates were not carried out after 60 cm.A high sand content (34%) at the core bottom(unit 3) was followed by an increase in silt(42–59%) and clay (38–42%; ?gure 2). Highest percentage of sand (38%) was observed in the middle of unit 2. The percentage of sand shows a marginal decrease in unit 1 and a minor increase towards core top (?gure 2). The lithogenic elements(Fe, Ti, Al, K and Rb) show highest concentrations in unit 3, decrease in unit 2 and a marginal decrease but near uniform concentrations in unit 1(?gure 3). On the other hand, the CaCO3and bio-genic elements concentration were low in unit 3, an increasing trend in unit 2 and reach maximum but near uniform values in unit 1 (?gure 4). TOC temporal variation closely follows the sediment texture being low in the sand and silt layer (0.17–0.50%)and high in the clay layer (0.27–0.92%; ?gure 4)of unit 3. The sandy sediments in unit 2 contain the lowest TOC concentrations (0.32–0.36%). The redox sensitive elements (Fe, V and U) show sharp, narrow and broad peaks between 51 and 55 cm in unit 3 (?gure 5).Detrital magnetic mineral concentration parameters ?LF, ?ARM , SIRM, Soft IRM and HIRM
generally display:
•relatively low values in unit 3 reaching lowest
values between 53 and 55 cm;
•a gradual increase from the base of unit 2
followed by sharp increase towards top of this
unit; and
•near constant values in unit 1 (?gure 6).The concentration independent ?FD% show low values at the bottom of unit 3 but shows a sharp decrease at 55 cm coinciding with ?LF .This decrease was accompanied by slight increase in the remaining part of unit 3 along with ?ARM /?LF and ?ARM/SIRM. The magnetic grain size parameters such as ?ARM/?LF and ?ARM/SIRM show a signi?cant decrease in unit 2 but near constant in unit 1. In contrast to uniform Soft IRM and HIRM in units 3 and 1, unit 2 shows a gradual increase from the base to a sharp rise to the top of the unit.S-ratio shows a sharp decrease at 55 cm. Total REE (?REE) concentration ranges between 140 and 192 ppm with an average of 156 ±12 ppm (?gure 7a). Average ?REE content was relatively higher in unit 3 (166 ppm) compared to units 2 and 1 (153 and 148 ppm; ?gure 7b).The Lan/Ybnratio was relatively high in unit 3(1.0–1.3) but low and uniform (?1) in units 2 and1. Shale-normalized REE patterns in all the units
show a small but a distinct positive anomaly.

Conclusion:
The emphasis of Himalayan studies is on understanding the sequential uplift of lithotectonic terrains and their erosional history via its signal as preserved in the Cenozoic foreland basin and Bay of Bengal sediments.
The evidence for a Miocene paleo Ganga is not as clear or direct as it is for a paleo Brahmaputra, but that is because of a lack of detailed studies. Provenance work on those Miocene mega channels in west Bangladesh and seismic profiling of the Gangetic plains should surely reveal a Miocene Ganga. Presently the Ganga breaches the Siwalik hills at Haridwar. In the past before the Siwaliks existed, beginning around 20 million years ago, there would have been a large mountain stream originating from the great heights of the Greater Himalaya, flowing through the expanse of the Lesser Himalaya and onto the plains somewhere near present day Haridwar, and then funneled by the elongate foreland trough, taking a southeasterly course towards the Bay of Bengal.