Reply to Discussion: Hennig-Breitfeld, J., H.T. Breitfeld, R. Hall, M. BouDagher-Fadel, and M. Thirlwall. 2019. A new upper Paleogene to Neogene stratigraphy for Sarawak and Labuan in northwestern Borneo: Paleogeography of the eastern Sundaland margin. Earth-Science Reviews 190, 1–32

Abstract

We thank Peter Lunt for his comment and his recognition of the importance of our new work. We welcome the opportunity to discuss further the implications of our new field and analytical data that led us to a view of the stratigraphy of Sarawak and Labuan on land different from his. In contrast to Lunt's statements, we do not think that our stratigraphic model is in conflict with much published work. Below we provide additional information to clarify and illustrate our new interpretation of the tectono-stratigraphic evolution of northern Sarawak in contrast to that presented in his Fig. 1. The comment illustrates the need for further research in the region. Many of the issues raised by Lunt are based not on new data, but on interpretations of literature from the 1960s, correlations from distant Sabah exposures or offshore wells, or unpublished industry reports. For example, Reinhard and Wenk (1951) are cited for identifying "the most important Tertiary orogeny of North Borneo" from an angular discordance reported in Sabah, apparently supported by unpublished comments by Bowen and Wright (1957), although in Sarawak Wolfenden (1960) commented that the "stratigraphic evidence is difficult to reconcile with the concept of an Upper Eocene orogeny that caused the entire… …Rajang Group to be folded". However, this misses the point of our paper, which was based on our own new field observations in Sarawak and Labuan and reinterpreted stratigraphic relations there. While we agree that previous field-based studies (e.g. Kirk, 1957; Wolfenden, 1960; Liechti et al., 1960) provide valuable information, it is also important to be aware of their uncertainties and limitations. Geological concepts have changed over the years (e.g. Kirk (1957, Plate VII) illustrates ripple marked Rajang Group sandstones captioned as "deposited in shallow-water marine conditions" which are now regarded as deep marine turbidites), biostratigraphic ages and time scale markers have been revised, and mapping was largely based on field observations limited to rivers combined with interpretations based on aerial photographs, whereas nowadays high resolution radar imagery provides significantly improved insights into structural relationships, seeing through vegetation cover, and GPS devices enable more precise location of outcrops. We presented new U-Pb age data from igneous rocks, biostratigraphy, and sediment data (light and heavy mineral compositions, U-Pb detrital zircon analysis) to determine maximum depositional ages and identify sediment provenance (Hennig-Breitfeld et al., 2019). We contend that the new data support major unconformities on land at c.37 Ma and c.17 Ma which are not those identified by previous workers. Our data also identify major changes in provenance in the Early Oligocene and late Early Miocene, in contrast to Lunt's claim that "widespread outcrops of Oligocene and Miocene clastic formations share a common sediment provenance". In our paper we drew upon previous literature concerning hinterland uplift and coastline reorientation,...

Provenance of Pleistocene Sediments in West Sarawak and Evidence for Pliocene Acid Magmatism in Central Borneo

“…Witts et al, 2012;van Hattum et al, 2013;Galin et al, 2017;Hennig-Breitfeld et al, 2019). There are few older zircons in the analysed sample, another feature of Schwaner origin rocks (Breitfeld et al, 2020).…”

Discovery of the marine Eocene in the northern South China Sea

“…If the SCS opening started from the east and then propagated westward since the Eocene with its oldest oceanic crust already subducted beneath the Manila trench, the newly discovered Eocene deposits can be considered as evidence to support an Eocene initiation of the SCS basin. But, if the Proto-SCS persisted into the Eocene, then these deposits would represent its northern margin marine record in an Eocene Proto-SCS that also sustained in its southern part the accumulation of such strata as the deep-marine turbidite Rajang Group [ 17 ] and Crocker Fan sandstones [ 18 ] in northern Borneo. Much further work is needed to clarify these issues.…”