The Permian-Triassic transition in the Blue Nile Basin: insights from petrography and geochemistry of sandstones
Abstract. The Permian-Triassic transition was a time of climatic, tectonic as well as ecologic reorganizations at a global scale. Clastic sedimentary successions are a key archive to study the response of earth surface processes to such extensive disruptions on land. Here, we focus on the so-called Fincha Sandstone, a Karoo-equivalent fluvio- to lacustrine succession of Permian to Triassic age, deposited in the Blue Nile Basin of central Ethiopia. We use thin-section petrography, bulk-rock geochemistry, and heavy mineral spectra in order to decipher source rocks, as well as the possible contribution of chemical weathering and recycling. The results show distinct difference between Early Permian and Late Permian to Late Triassic sandstones.
Petrographically, the Early Permian sandstones are rich in feldspar, and unstable heavy minerals like apatite and garnet. The average chemical index of alteration, trace, and rare earth element concentrations suggest little chemical weathering and relative proximity to the source area. This was controlled by a high topography, alluvial and braided systems and a semiarid climate.
In contrast, the Late Permian and Late Triassic sandstones are quartzose, have a lower content of feldspar, and show ultra-stable heavy mineral assemblages rich in zircon, rutile, and tourmaline. The chemical index of alteration is around 80 to 90 %. This can be explained by a combination of recycling of mature platform sediments together with a humid climate reflected in deltaic-lacustrine deposits. Most probably, a corrosive environment around the Permian-Triassic Boundary has further fostered high sediment maturity.
In the Middle to Late Triassic, sandstones become gradually immature again, marked by a significant increase in lithoclasts and metamorphic heavy minerals such as garnet. This trend is less visible in geochemical data because lithoclasts are fine-grained quartzose and derive from low metamorphic terrains typical for the basement of the Arabian Nubian Shield. This makes the onset of axial, NW-SE directed sediment transport through the Blue Nile Rift Basin most probable.
This preprint has been withdrawn.
Maryam Mansouri et al.
Maryam Mansouri et al.
Maryam Mansouri et al.
Viewed (geographical distribution)
The paper from Mansouri et al " The Permian-Triassic transition in the Blue Nile Basin: insights from petrography and geochemistry of sandstones" has the potential to contribute to the reconstruction of the Permo-Triassic puzzle, which is indeed of global interest. The analytical tools are mostly appropriate and should be enough to unravel the major changes in the Blue Nile Basin.
However, the paper is more than problematic. I'll try to summarise the main issues here below, while I provide more detailed comments in the attached PDF.
The English is more or less ok, although sentences are very lengthy and sometimes confused. The issue could be easilty soleved during revision.
The structure and content of the paper is problematic.
The Introduction is obsolete, it reads like the paper was written 10-15 years ago.
I found the chapter on the Geological Setting to be surprisingly limited in relation to the main aim of the paper. While it reviews the evolution of the area from much deeper time, important information functional to the paper is only dropped in the Discussion chapter. As a result, most of the content of the geological setting is not really used in the Discussion. Conversely, the author has to deal with plenty of information which are also provided in a poorly structured way.
From a technical point of view, there are several problems from both technical and conceptual point of views.
Sedimentary Petrorgraphy is extremely poor. Apparently, there is a problem in distinguishing textural from compositional maturity (the latter is the one which is used more by the authors). There is no distinction between plagioclase and K-feldspar, which is very odd, even more considering the objective to determine provenance and chemical weathering. Lithic and rock fragments are not characterised at all, even in the case of metamorphic fragments which are particularly important for provenance.
Heavy minerals are ok, although the graphic representation is pretty poor. Is not clear why they decided to not use biplots rather than obsolete or very subjective plots. Differences in composition could also be explained by weathering, hydraulic sorting, and diagenesis, however, the authors do not bother at all to explore alternatives to their interpretation. It is stated that samples are either medium- or fine-grained. This could determine a lot of biases, another problem which is not tackled by the authors.
The way geochemistry is used is also very obsolete. It has been proved by an enormous amount of literature that both CIA and McLennan-Taylor-Bathia-Crook do not work, even more for sand and sandstone. Most of the interpretation is based on this data, which is really worrying.
The Discussion is the most problematic part. Is very disorganised, cumbersome, and redundant. The way compositional signatures are used is almost the same as it is in the Result section. Importantly, skepticism increases considering the non-consideration of processes that can modify the detrital composition.
AS mentioned before, there are much more issues which I have highlighted in the attached PDF.
Considering all the issued, I don't think a thorough revision is achievable in the timeframe allowed for resubmission. I would recommend a rejection with reinvitation to submit.