Please excuse my long break in posting the next contribution to this mini-series (Part 1 and Part 2). As you have read on my blog I was stuck in the field with lots of snow and other more urgent matters to attend to. Back home I was compiling my few preliminary results to present them.
In a note ahead I recommend reading the two articles referenced at the bottom for details. The topic is rather extensive and my post here is only a small glimps of the issue and far from a complete synthesis. Nevertheless I believe it will suffice to broaden my own and everybody's geological horizons.
Today I would like to discuss some concepts of the paper by Pawellek and Aigner called Dynamic stratigraphy as a tool in economic mineral exploration: ultra-pure limestones (Upper Jurassic, SW Germany) and it's implications for sequence stratigraphy and industrial minerals. Ultra-pure limestones are the resource for many industry uses in the manufacture of glass, paper and many other applications.
Sequence stratigraphy has long played an important role in predicting prospective regions or formations in the hydrocarbon industry, however, it can also be used for other, perhaps less known, approaches in the raw-material exploration.
Knowing the genesis of deposits greatly reduces the time and cost of exploration by focusing only on the most prospective areas. Pawellek and Aigner use concepts of "dynamic stratigraphy" for this, ranging from microfacies analysis, sequence to basin analyses to understand the occurences of ultrapure limestones. They are using reef and particle-rich limestones from the Upper Jurassic as an example.
Microfacies analysis is a good tool to determine depositional environments and the effects of diagenesis and thus also provides information on both the chemical composition and economic potential. For example low-energy carbonate rocks usually have a higher content of clay or other, unwanted matter and are less pure than high-energy rocks like grainstones.
Petrophysical and facies analysis focuses on the chemical-physical properties and is used in characterising the quality.
Architectural analysis allows the reconstruction of shape and volume of a deposit.Sequence analysis highlights the genetic framework and occurence within sedimentary sequences and cycles. Among other things this reveals the levels within the stratigraphic record where economic mineral concentrations could most likely be present.
Last but not least the analysis of stacking patterns and basin analysis allows us to determine regional concentrations and assess the basin-wide scale. A direct step to predicting the possible distribution of profitable facies.
In their paper Pawellek & Aigner examine two major facies of the Upper Jurassic. The so called "normal facies" and the "massive facies". The former consists mainly of well-bedded limestones and calcareous marls, the later has no or at least a very inconsistent bedding and is host of extensive reefs, mounds or bioherms.The two major facies are further divided into sub-categories based on their composition. Within the bedded limestones the particle-rich limestones composed mainly of intraclasts, peloids and ooids are the most pure. In the massive limestones the purest are the coral debris limestones and the stromatolites (+thrombolites). Both with an average CaCO3 content above 98% and reaching-up to 100%. These also have the highest witness index values.
As a sidenote: Due to the variable clay content these rocks have different levels of radioactivity with the purest limestones having the lowest activity and the clay-richest the most active. They show how this implies a correlation between gamma-ray count and facies types.
In sequence analysis facies belts can not only be recognised on a regional scale but also facies zones can be identified within outcrops, cores or logs. Shallowing and deepening would be present in vertical successions of regular cycles. This can be observed.Shallowing-upward and deepening-upward halfcycles show symmetrical facies successions. Considering a wide range of evidence like clay content, organism successions, matrix and ooids content and other evidence this can be attributed to changes in water-energy and waterdepth. As an example upward increases of ooids, size, roundness and sorting can be interpreted as an increase in depositional energy.
Cycles of meter-scale thickness (4 to 10m) can be regionally correlated in Southern Germany. These cycles are referred to as "genetic sequences" that record phases of shallowing followed by deepending. Seven of these have been identified in the Upper Jurassic of Southern Germany. Within these "genetic sequences" ultra-pure limestones are most abundant in zones of highest depositional energy. Clay and other impurities are prevented from settling in these high-energy environments. In general this results in the bioherms and particle-rich limestones to be the purest.
To put this into a sequence stratigraphic framework:
When and where do bioherms and particle-rich limestones - reefs, stromatolites and peloid-ooid-pack- or grainstones form? As outlined we do need a high-energy depositional system with little clastic input and shallow waterdepth. When we think of the succession of depositional environments during sea-level rise and fall the above mentioned criteria are most usually met within the transgressive system tract at the shelf- or platform edge in the form of shoals or reefs. When sea-level rise continues we can expect the development of hardgrounds which are often concentrated along the maximum flooding surface. The production of large amounts of pure carbonate would cease.
We learned (and me, too): The most prospective areas for ultra-pure limestones are genetic sequences of shallowing-upward within the transgressive system tract that show distinct evidence of shallow, high-energy depositional environments close or on the shelf or platform edge.
- Thomas Pawellek and Thomas Aigner (2004): Dynamic Stratigraphy as a tool in economic mineral exploration: ultra-pure limestones (Upper Jurassic, SW Germany), Marine and Petroleum Geology, 21
- C. Robertsen Handford and Robert G. Loucks: Carbonate Depositional Sequences and System Tracts - Respondes of Carbonate Platforms to Relative sea-level change, in Loucks, R, G. and Sarg, Rick, eds., Carbonate sequence stratigraphy: Recent advances and applications: American Association of Petroleum Geologists Memoir 57