Hi everyone! I'm working on sequence stratigraphy of shallow-water marine carbonates from the Lower Cretaceous of Spain. I've been trying to find an explanation for a specific sample's texture for a while now. It's taken from a 2 meter thick cross-stratified bioclastic limestone level capped by a 0,5 m thick, intensely bioturbated horizon which I have identified as a regionally correlatable parasequence boundary. Other samples from different (yet also sequentially relevant) positions show similar textures.
At first I attributed the sample's intraclastic-lithic peloidal patches to storm reworking of a semiconsolidated substrate. However, an old publication dealing with the same stratigraphic section I'm working on states than this level "shows classic features of meteoric diagenesis, evidencing subaerial exposure", without naming or describing any of said features. I'm a bit confused, as I haven't been able to identify pendant or meniscus cements, or any other morphological feature indicative of vadose/phreatic cementation.
Are there any features I may have overlooked? Can anybody shed some light on the genesis of this sample's texture?
The first picture is a scan of the whole thin section; the second is a detail view. The third picture is a detail of a second sample from a different stratigraphic section with an apparently similar texture.
Dear Mr. Garcia-Penas,
I have worked in sedimentology applying sequence stratigraphy to shed some light on the origin of mineral deposits and also attended the course on microfacies analysis by the late professor E. Flügel. But I hesitate to speculate on your setting based on my knowledge and experience gathered in these case histories of economic geology. I would like to direct your thoughs to his textbook Microfacies of Carbonate Rocks-Analysis, Interpretation and Application (E.Flügel 2010). I can imagine that in this textbook and some solid information drained down from a textbook on seqeunce stratigraphy you should come to grips with this enigmatic facies built-up in your study area.
I wish you much success
H.G.Dill
p.s. DILL, H.G. (2012) Diagenetic and epigenetic mineralization in Central Europe related to surfaces and depositional systems of sequence stratigraphic relevance.- In: Ketzer, M., Morad, S. and De Ros, L.F. (Eds.), Linking Diagenesis to Sequence Stratigraphy of Sedimentary Rocks, International Association of Sedimentologists (IAS), Special Publication, 45: 151-182 (available on the RG server)
Thank you, Mr. Dill.
I'll be sure to check out prof. Flügel's textbook.
I'd accept that intraclastic-lithic peloidal patches are due to storm reworking of a semiconsolidated substrate and move on.
Dear Álvaro,
I well appreciate the recommendation from prof. Harald, the Flügel's book will absolutely give you the respective description and interpretation of this kind of microfacies and related texture. By the way, I have seen similar microfacies from the Mid-Cretaceous carbonate platform of Central Tunisian Atlas.
Khaled Trabelsi Norman Malcolm Mcclure I think storm reworking is indeed the most parsimonious explanation given the available data. Thank you!
I went through some notes in my field notebook containing my observation in Oman, where I found a similar scenario in the Cretaceous Natih Fm. in a well log where series of grainstones-rudstones-floatstones marked the transition from an inner to the mid ramp setting. Another setting which I can your photography compare with is a clinoform with abundant rudists in grainstones marking a sequence boundary which upwards converts into a zone of emersion, reworking, and karstification while towards the opposite direction an incised valley fill developed as a consequence of a lowering of the sea level. I am not a paleontologist but a field geologist and economic geologist by profession.
It is more a set of ideas which you can meditate upon and see if these suggestions match with your overall observations in the field and the laboratory.
I wish you much success H.G.Dill
...important for the depositional setting are also the occuring (micro)fossils...I notice serpulid tubes in all three images.
Hi Alvaro
Based on lots of shell fragments, bryozoa, gastropods, worm tubes,etc. These facies may belong to shallow marine settings and such settings are sensetive to see level fall, one may recognize some signs of bioturbation ad well. However the nature of micrite looks like sedimentary and not chemical, and chaotic nature of biofragments juxataposition indicates high energy processes such as storm.
There is no sign of grain dissolution as well, there was no time and stability for any sort of cementation to form. Even if these sediments are exposed to metroric processes later ( which is unlikely) the weather should be interpreted as warm and dry not humid.
I hope it helps
Take care
Hesam
