The presence of calcite indicates the alkaline condition of the Basin while the quartz or silica reflects acidic condition of the sedimentary environment. you can also support the thin sections data with XRD. This means that it is an artifact of sea level fluctuations, further more you support the data with some mineral like glauconite indicates marine condition if you have recorded.  

Dear Saeed,

Thanks alot for your comments. Actually, there are alternative beds/laminations of sandstones; some of which are rich in calcite while the others in quartz. My opinion is also same as your that  there is sea level fluctuation but some of the sedimentologists  disagreed and said that this calcite is diagenetic. My point is that  definitely the calcite occurs as a diagenetic product but not as enough as is in our samples, where it even exceeds to quartz and in some samples it reaches up to 50%. That is my concern.

Is the calcite cement or detrital grains?  If it is cement, it won't be useful as an environmental indicator.  Sandstone texture, sedimentary structures, and stratal geometres are far more useful in environmental interpretations.  If the calcite actually are grains that accumulated with the siliciclastic grains, then you should be able to figure out whethere they are skeletal grains, oolites, rock gragments etc.

I agree with Prof. Malone. Their are other tools to determine the depositional environment such as texture and sedimentary structures. Detrital minerals are more useful in provenance studies. Of course glauconite or some  chemically and biochemically precipitated primary minerals suggest pH and Eh and bathymetry to some extent.

Presence of calcite in high proportion in the sand tone, indicates two possibilities

1) if its truly sandstone, then  the quartz clasts are bound by calcite cement, which seems to be quite probable case where we can get calcite quartz combination, which I have observed in many sandstones where even the original clasts of quartz /feldspar are replaced by calcite, which is diagenetic in origin and nothing to do with the depositional environment.

and the other possibility of the chemical analysis could be

2) a limestone bearing arenaceous impurity represented by presence of quartz and feldspar clasts, which definitely indicates fluctuating basinal conditions wherein the coarse clastic sediments are getting deposited when a limestone is in formation.

A single photomicrograph of the rock will solve all the problem, if you have thin section photo please add, that will solve all the doubts. Whether the calcite is cement or clast.

Dear Muhammad,

I find your question interesting, having calcareous sandstones and siliceous limestones. Limestone and feldspars do not stand long distance transport both because of the cleavage and solubility.

Petrography will show the mineral textures, whether  framework, cement or matrix. Also the inclusions in the framework grains can tell the provenance. If the carbonate is present as cement it is diagenetic. The same applies for quartz. Point contacts and pressure solution textures in quartz distinguish the derivatives from source from diagenetic quartz which will occur as cement. and overgrowths. Are the feldspars undergoing sericitization?

Note that depositional environment can be syn- or post-  and diagenesis occurs in the post-depositional stage, i.e. which all the same is depositional.

The type of feldspar is also important because the more calcic feldspars are unstable in the sedimentary environment and can  indicate the provenance of the sediment while those in the albite range can derive from diagenesis. Muscovite also derives from diagenesis as well as from acidic rocks. Some of the carbonates could even result from alteration of the calcic feldspars and other carbonates, and giving clays as the end product as you obtained.

Well done.

Obianuju P. Umeji

Dear David Malone,

Thanks alot for your reply and sharing information. The calcite is not skeletal or granular. It is as a cementing material. The  XRD, Petrography and EDX indicate that in a few sandstones; the calcite is even higher than quartz. It is actually the problem that is it possible that the diagenetic calcite may be present in certain sandstones up to such high % as are our samples. If answer is in yes then how will we classify such sandstones (I mean which classification is suitable). Hope that you have understood what is my point of view.

Dear B.P. Singh,

Thanks for comments.Can you please justify such huge concentration of calcite in a sandstone?

You must study thin sections under microscope to answer this type of questions.

Dear Muhammad,

The answer takes one back to the question concerning the provenance of the sandstone. Since the calcite is in the form of cement, it was precipitated from solution. Sources of calcite could come from dissolution of former calcium rich minerals such as calcic plagioclase, Ca-pyroxenes, usually of igneous source, as well as carbonate sediments. Also since the framework elements are quartz in minor quantities, consider occasional transport of quartz  into a carbonate shelf, away from clastic influx, i.e. Asoori's No. 2  answer as above.

Are the quartz strained, showing undulose extinction (metamorphic) or not?, Tells if the source is from basement or older sedimentary source.  

What sedimentary structures are associated with the deposits?

All the best.

Obianuju P. Umeji

Each mineral with different composition has a unique density. Is there any relationship between the mineral density and environment of deposition? As a layman I say that higher density minerals should have a place in the environment like delta and therefore gradual decrease in density may predict a shift in the environment of deposition towards deep see.

regards

Ijaz 

Dear Muhammad,

Understanding the sedimentary environment from the mineralogy only is visrtually impossible, unless you don't have a calibrated data set. Understanding of how hydraulic sorting works in different environments is of course an advantage. Meandering rivers are great "selectors" and once you get the pattern is normally quite easy to use geochemistry and mineralogy to predict facies and/or environment distribution. About the calcite topic, alternating bands is quite a common deatures and normally results from the interplay between grain-size - sorting - fluid composition - sub-aerial exposure. Examples that may be helpful can be found at the attached links.

Hope this helps,

Luca

Article Diagenetic constraints on heterogeneous reservoir quality as...

Article The role of detrital anhydrite in diagenesis of aeolian sand...

Dear Luca,

Thanks for information. Yes, you are right that only mineralogy is not sufficient to decide about the depositional environments. Along with huge data set such as the petrography, XRD, SEM-EDX; we have also the control of field data such as the facies analysis etc. In the field data, we have the alternative beds of shale/clay, sandstone from a few feet to meter scale having variable proportion of calcite as a cementing material. Along with this, the sandstone has  cross stratification, wood logs, bioturbation, hummocky cross stratification, some gastropod fossils, thin gypsum lamination, vertebrate fossils etc. My concern is that the units having such huge concentration of calcite in the form of calcite along with bioturbation, hummocky cross stratification and gastropods may belong to transitional environments such as the beach, deltaic etc while the units having low calcite cement along with cross stratification, wood logs, vertebrate bone fossils belong to fluvial system but a few geologists oppose it and their point of view is that all the above features are common in both of the environments and one may not differentiate that which one is transitional and fluvial.

mineralogy alone can't answer, as the depo. eviront. is controlled by many other factors.

Interpretation of depositional environment of sedimentary rocks is not limited to a certain aspect. It has many other controlling factors besides mineralogy. In sandstone petrography, detrital mineralogy can add clues to their depositional environment. Semi-quantitative data obtained from XRD can also help in interpreting depositional environment. Indeed, it's a collective role of certain aspects incorporated in these rocks which in turn can help in determining their depositional history.

Fine. Thanks for your input.

example the study of sandstone are principally composed of framework grains , matrix and cement , in addition to minor amount of accessory minerals.the variation in ratio and type of lithic fragment with differences in type and mineral percentage depending on variation in sedimentary environment and mineralogical composition of source area.

it is not that simple.critical observation required.