The Basin of Meulaboh is the coastal alluvial basin of the Meulaboh district on the West coast of Atjeh, distinguished in the colonial geological literature as the type-area for the late-Tertiary to Recent geological evolution of the western Atjeh coast, and as the locus of the only colonial-era gold-dredging operation on Atjeh territory — the M.A.E.M. dredges that began work on the Geudong Concession in 1940. The basin’s geological structure, surface form and alluvial gold-and-platinum content were first systematically mapped by Zwierzycki in 1922 on sheet I of the 1:1,000,000 geological map of the Netherlands Indies, and the basin’s mineral geography was subsequently re-described by De Groot (1946) and synthesised by Van Bemmelen (1949).
Geological structure
Van Bemmelen reproduces, as Fig. 28 of The Geology of Indonesia, the Zwierzycki 1922 map of the Basin of Meulaboh. The map distinguishes:
- Alluvium — the recent alluvial coastal strip, at present subsiding, which shows no river terraces.
- Neogene (Pliocene, Miocene) — the late-Tertiary marine sediments of the coastal embayment.
- Paleogene (Oligocene, Eocene) — the older Tertiary sediments exposed along the inland margin.
- Permo-Carboniferous — the pre-Tertiary basement of the central mountains.
- Andesitic Effusiva, Andesite, Basalt (B), Diabase (D), Gabbro (G), Serpentine, Granite — the igneous and metamorphic complexes of the interior.
Van Bemmelen adds a critical correction: the “Losir (Löser)” indicated on Zwierzycki’s map as a volcano is “not volcano, as has been indicated on this map, see volume I”. With that caveat, the map remains the standard cartographic basis for the surface geology of the West-Atjeh coastal plain.
Surface form: terraces and the sudden southward bend
The most striking surface feature of the basin, as Van Bemmelen describes it, is the system of pleistocene river terraces that “begin directly at the foot of the central mountains and extend downstream to the sudden southward bend in all larger rivers”. The terraces are “pleistocene plains of river deposits, dissected by recent uplifts and rejuvenated erosion”, and the sudden southward bend in the larger rivers “are most probably the result of a young fault zone, parallel to the coast. The faulting movements are recent or subrecent, because traces of the former river courses, forming the western extension of the upper courses, have been observed in the alluvial lowland.”
The alluvial coastal strip itself, by contrast, “is at present subsiding, and shows no terraces”. In the upper course of all larger rivers only one terrace has been found. The basin is therefore a tectonically active landscape: the inland terraces are Pleistocene remnants of an older, more seaward-extended drainage system, while the present alluvial coastal strip is subsiding and receiving the modern sediment load of the rivers.
Gold and platinum placers
The pleistocene terraces of the Meulaboh basin are the principal repository of the alluvial gold of the West coast of Atjeh. Van Bemmelen records that “the alluvial gold at sites of second-, third-, or higher order of displacement is found in the river terraces between the central mountains and the alluvial lowland. The gold grains are coarse near the mountain border, becoming smaller downstream. The gold-bearing black sands contain fine grains of cinnabar, besides considerable amounts of platinum.”
The terrace deposits are “from ten to hundred feet thick”, and “remains of these terraces, in so far as they have not yet been worked by the natives, often proved to be very rich”. The primary sites of the gold “have to be sought in the pre-tertiary Old Slates Formation and granite contacts of the interior”, but “due to the outbreak of the war these primary deposits have not yet been explored.”
The M.A.E.M. dredging operation
Production from the Meulaboh placers began in 1940 on the Geudong Concession of the M.A.E.M. The operation used two bucket dredges (buckets of 6 cubic feet) with a joint capacity of 270,000 cubic yards per month. The deposit contained 55–60 per cent boulders (the largest of which “could just be handled”) and 40–45 per cent fine material. There were two types of gold: fine gold with somewhat higher silver content, which was easily amalgamated, and coarse gold with lower silver content, which was difficult to amalgamate. The platinum production was some kilograms per month. Treatment was by amalgamation and Wilfley tables.
De Groot’s 1946 report observed that the M.A.E.M. dredges, in 1941, used 400 kg of mercury per bucket dredge per clean-up to recover the finer gold, and that “after every clean-up the quantity of mercury had increased” — the drilling samples occasionally contained native mercury, and the black sand contained very fine grains of cinnabar. This is the only documented colonial-era observation of native mercury in Atjeh alluvial deposits; see Mercury in Atjeh.
Significance
The Basin of Meulaboh is significant for the encyclopedia of Acheh history on three grounds. First, it is the type-area for the late-Tertiary to Recent geological evolution of the western Atjeh coast, and the Zwierzycki 1922 map of the basin is the standard cartographic basis for all subsequent geological work on the region. Second, it is the locus of the only colonial-era gold-dredging operation on Atjeh territory — the M.A.E.M. dredges of 1940–1941 — and accordingly the only Atjeh locality for which colonial-era gold production figures are recorded. Third, the basin’s alluvial deposits are the only Atjeh locality for which native mercury and cinnabar have been documented in the colonial geological literature.
See Also
- Acheh
- Atjeh’s Westkust
- C. de Groot
- Geudong Concession
- Gold and silver in Atjeh
- J. Zwierzycki
- M.A.E.M.
- Meulaboh
- Mercury in Atjeh
- Platinum in Atjeh
- R. W. van Bemmelen
- Van Bemmelen’s The Geology of Indonesia (1949)
- 1940
- 1941
- 1946
Source
The Geology of Indonesia, by R. W. van Bemmelen, Vol. II: Economic Geology (The Hague: Government Printing Office; Martinus Nijhoff, 1949), pp. 121–122 (Basin of Meulaboh, Fig. 28, and gold-placer description), p. 223 (West coast of Atjeh mercury). The map: “Basin of Meulaboh (West coast of Atjeh). According to the geol. map 1,000,000, sheet I, Compiled by ZWIERZYCKI, 1922.”