Depositional Rate of the Paleogene Nanggulan Formation
Kulon Progo Mountains, Yogyakarta Special Province
GAYATRI, INDAH MARLIYANI and RAHARDJO, WARTONO
Department of Geological Engineering, Gadjah Mada University, Yogyakarta,
Indonesia
The Nanggulan Formation crops out along the eastern flank of the West
Progo Mountains, 30 km west of Yogyakarta. This formation is one of the very
limited Paleogene outcrop in Java island. Based on the fosil content, the
formation is subdivided into 4 divisions, which from the older to the younger are
: Axinea, Nummulities, Discocyclina, and Globigerina Division or Beds.
Systematic sampling shows that Axinea-Dyscocyclina Bed were deposited
during the P11 (Middle Eocene, 45 mya) on a transitional to Middle Neritic
environment, with thickness of more than 600 meters.
Another detailed measurement and sampling has been done on the
outcrops of the upper Nanggulan Formation, Globigerina Bed, in Balak village.
The age of this upper part ranges from P11 to P22. The lithologic column in this
area can be subdivided into 7 planktonic foraminiferal zones based on first and
last appearances of datum species, which are : Globigerinatheka
subconglobata curryi - Globigerinatheka subconglobata eugaena zone (P11),
Globigerinatheka subconglobata eugaena - Orbulinoides beckmani zone (P12 ),
Orbulinoides beckmani zone (P13), Globigerinatheka semiinvoluta zone (P15),
Globigerinatheka semiinvoluta - Turborotalia cerroazulensis pomeroli zone
(P16/17), Globigerina venezuelana - Globigerina ampliapertura zone (P20),
Globigerina binaiensis - Globigerina ciperoensis fariasi zone (P22). Based on
the benthonics it is interpreted that the upper part of the Nanggulan was
deposited on Bathyal to Abyssal depth, with thickness of only 30 meters. This
condition implies a sudden change on depositional rate. The depositional rate
of the Axinea - Discocyclina Beds is approximately 600 meters per million years
while the Globigerina Bed has only 1.2 meters per million years.
The differences on the depositional rate is parallel to the change in
paleobathymetry, as shown by the benthonics, from shallow water Axinea –
Nummulites Beds to deep water Globigerina Bed.