I have a question about provenance in siliciclastic rocks. In the Early Paleozoic succession of the Taebaeksan Basin, Korea, there are sandstone-dominated Jangsan Formation and shale-dominated Myobong Formation, overlying the Jangsan Formation. The relationship between these two formations remained problematic. It has been thought that these two formations were conformable. However, due to poor outcrop, it is not easy to carefully observe the contact between the formations. Trilobite data suggest that the Myobong Formation deposited during the late Cambrian Series 2 (514~509 Ma). No fossil has been found from the Jangsan Formation.
One recent study (Lee, Y.I. et al., 2012, sedimentary geology) shows that the distribution of detrital zircon significantly differs in the lower Jangsan Formation and upper Myobong Formation (I attach figure below). Based on that, they suggested there is a major unconformity between these two formations. According to the zircon age dating, the Jangsan Formation yields zircons of 1738±67 Ma to 3058±53 Ma, but the Myobong Formation yields 358±12 Ma to 2691±125 Ma. Lee et al. (2012) ascribed the zircons younger than 509 Ma to lead loss.
Personally, I doubt whether they are right or not. First, the age of youngest zircon can only indicate lower limit of depositional age, which can differ greatly from actual depositional age. Second, as Lee et al. (2012) noted, there is major lead loss in the Myobong zircons, which may cause problems in age dating. Third, detrital zircon age data from the Myeonsan Formation (Kim, H.S. et al., 2013, EPSL), which is thought to be an equivalent of the Jangsan Formation, contains an age of 512±6 Ma.
I like to know whether there is any works similar to this example or not. Is Lee et al. (2012)'s data are enough to argue that there is an unconformity? If not, is there any other example to compare?
References:
Lee, Y.I., Choi, T., Lim, H.S., Orihashi, Y., 2012. Detrital zircon U–Pb ages of the Jangsan Formation in the northeastern Okcheon belt, Korea and its implications for material source, provenance, and tectonic setting. Sedimentary Geology 282, 256-267.
Kim, H.S., Hwang, M.-K., Ree, J.-H., Yi, K., 2013. Tectonic linkage between the Korean Peninsula and mainland Asia in the Cambrian: Insights from U–Pb dating of detrital zircon. Earth and Planetary Science Letters 368, 204-218.
Surely. It may have occurred due to sediment influx from two different fluvial systems, but have debauched at two different geographically separated locations in a large sedimentary basin. In the Santonian-Campanian strata of the Cauvery basin, similar feature is recorded. Spatio-Temporal variations of REE patterns have also confirmed this feature and is under publication.
Ramkumar, M., Subramanian, V. and Stüben, D., 2005 Deltaic sedimentation during Cretaceous Period in the Northern Cauvery basin, South India: Facies architecture, depositional history and sequence stratigraphy. Jour.Geol.Soc.Ind. v.66. pp.81-94.
Dear Lee:
1. Provenance variations can be significant in most sedimentary formations and are controlled by un-roofing at the source rock.
2. From the data you have, the temporal events have a duration of 10's of million years if not more. This time frame is more than enough to have derivation from different provenances.
3. A very simple way to do this is using the basic petrographic techniques. Examine the composition of the sandstones from these two formations and the modal analyses should give you clues. I am sure the older literature has enough data on this!
Palynofacies can aid your request ,by analize the core samples that give good indications of Palynostratigraphic and paleodepositional environments .
Greetings, I have similar experience from the Upper Paleozoic siliciclastic system of central European Variscides: - actually nonfosiliferous older infill is overlain by fosiliferous (e.g. goniatite zones of Upper Viséan) sequence of different source (HT and HP metamorphites). All that has been proved by different detrital garnet assemblages and reflects the moment when deep metamorphites were tectonically unroofed and began to suply the basin...morre see Hartley - Otava 2001.
Stating that there is an unconformity is not enough, it is always better to define it.
The data you present give you a clue but it is not enough to define the order and magnitude of the uncorformity.
Remember that it is not always simple and paraconformities are everywhere (ex: siliciclastic formations known to be conformable on subsurface analysis sometimes present disconformities on the field).
Remember also that British and American Geologist (and probably others) may disagree about the definition of what is a disconformity (structure Vs nonconformity).
I realized that I completely forget to answer the question.
Depending on the definition of disconformity that you will be using, two conformable formations can have different provenances.
I am actually working on a megasequence in Thailand that appear to be conformable on every single seismic profile made by petroleum scientist. However looking at the petrography, like Narender_Pendkar advice you, there is definitely two distinct petrographic profile. There is a shift of sediment origin inside the megasequence that is not clearly associated with an unconformity (even if there are probably paraconformities all along the sequence).
So I would say yes it is possible.
Are you able to observe a "transitional" facies between your formations?
Dear all, thank you for the answers. I also found one reference which shows variable sets of zircon ages (Horton et al., 2010, GSA Today) that changes both laterally and vertically. It seems that it is a common phenomenon.
Muthuvairavasamy Ramkumar: Thank you for the manuscript. It helped me a lot to understand about provenance.
Narender Pendkar: the time duration between the two formations can be less than a few million years, or more. I do not have skill for petrographic analysis, but I may find some references for it. Thank you for the advise.
Ahmed Al-Ahmed: I cannot do palynofacies analysis, because it is a Cambrian succession.
Jiří Otava: Thank you for the reference. I will read it carefully. But in this case, how can you prove that the unfossiliferous older infill deposited contemporaneously with the overlying deposits?
Romain Liard: there is no transitional facies between the formations, based on the limited outcrop observations. As I written above, outcrop is poor and it is not easy to observe it carefully. On the other hand, there is a strange structure between the two formations, which I think is soft-sediment deformation (which Lee Y.I. et al., 2012 argues as an evidence of unconformity).
Well we have both direct and indirect proves of paracontemporaneous deposition - the unfossiliferous older infill contains redeposited devonian and carboniferous limestone pebbles.
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