Reconstructing post-Jurassic overburden in Central Europe: New insights from mudstone compaction and thermal history analyses of the Franconian Alb, SE Germany
- 1GeoZentrum Nordbayern, Friedrich-Alexander University (FAU) Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany
- 2Geothermal Technologies, Technical University of Munich (TUM), Arcisstraße 21, 80333 Munich, Germany
- 3Department für Angewandte Geowissenschaften und Geophysik, Montanuniversität Leoben, Peter-Tunner-Straße 5, 8700 Leoben, Austria
- 1GeoZentrum Nordbayern, Friedrich-Alexander University (FAU) Erlangen-Nürnberg, Schlossgarten 5, 91054 Erlangen, Germany
- 2Geothermal Technologies, Technical University of Munich (TUM), Arcisstraße 21, 80333 Munich, Germany
- 3Department für Angewandte Geowissenschaften und Geophysik, Montanuniversität Leoben, Peter-Tunner-Straße 5, 8700 Leoben, Austria
Abstract. The Franconian Alb of SE Germany is characterized by large-scale exposures of Jurassic shallow marine limestones and dolostones which are frequently considered as outcrop analogues for deep geothermal reservoir rocks in the North Alpine Foreland Basin farther south. However, the burial history of the Franconian Alb Jurassic strata is not well known as they were affected by emersion, leading to extensive erosion and karstification with only remnants of the original Cretaceous and Cenozoic cover rocks preserved. To estimate the original thicknesses of the post-Jurassic overburden we investigated the petrophysical properties and the thermal history of Lower and Middle Jurassic mudstones to constrain their burial history in the Franconian Alb area. We measured mudstone porosities, densities, and maturities of organic material and collected interval velocities from seismic refraction and logging data in shallow mudstone-rich strata. Mudstone porosities and P-wave velocities vertical to bedding were then related to a normal compaction trend that was calibrated on stratigraphic equivalent units in the North Alpine Foreland Basin. Our results suggest maximum burial depths of 900–1700 m of which 300–1100 m are attributed to Cretaceous and younger sedimentary rocks overlying the Franconian Alb Jurassic units. Compared to previous considerations this implies a more widespread distribution and increased thicknesses of up to ~900 m for Cretaceous and up to ~200 m for Cenozoic units in SE Germany. Maximum overburden is critical to understand mechanical and diagentical compaction of the dolostones and limestones of the Upper Jurassic of the Franconian Alb. The results of this study therefore help to better correlate the deep geothermal reservoir properties of the Upper Jurassic from outcrop to reservoir conditions below the North Alpine Foreland Basin. Here, the Upper Jurassic geothermal reservoir can be found at depths of up to 5000 m.
Simon Freitag et al.
Status: final response (author comments only)
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RC1: 'Comment on se-2022-18', Thomas Voigt, 07 Mar 2022
Intersting paper with a consistent, well discussed data-set. The conclusions could be extended to some more results concerning paleo-heatflow and timing and rates of exhumation (pin-pointing time-span by surface geology).
More questions (for my own understanding) and some suggestins for better organisation of the paper are in the text. It is not necessary to sedn the revised version again to me.
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AC1: 'Reply on RC1', Simon Freitag, 12 May 2022
The authors thank the reviewer Dr. Thomas Voigt for the positive feedback and comments provided for our manuscript. Your suggestions helped in improving the content, readability and strengthen the interpretation. On your recommondation, we extended the discussion and conclusion with respect to the timing and rates of exhumation, where we compare and discuss published results to ours. However, our data did not allow for the estimation of paleo-heatflow, which needs to be investigated in upcoming studies.
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AC1: 'Reply on RC1', Simon Freitag, 12 May 2022
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RC2: 'Comment on se-2022-18', Hilmar von Eynatten, 06 Apr 2022
Simon Freitag and co-authors use petrographic and petrophysical properties and organic maturation data of Lower and Middle Jurassic mudstones from outcrops and drillcores of the Franconian Alb to estimate thicknesses of the post-Jurassic regional overburden. The paper is overall well written, methods and calibrations appear sound to me (though I’m not an expert in petrophysical properties), and the results constitute a significant and highly relevant contribution for the understanding of the Mesozoic evolution of the area. I recommend minor revisions only. The authors may consider separating chapter 3 into ‘Results’ (largely sections 3.1 to 3.4) and ‘Discussion’ (largely 3.5 and 3.6, could then be a new chapter 4).
When comparing the results to those by von Eynatten et al. (2021) in section 3.5, please consider that their modeling leading to 3-4 km burial refers to Early Triassic (Bundsandstein) strata (their figure 10). Including about 600-800 m of Middle Triassic (Muschelkalk) and Late Triassic (Keuper) strata significantly reduces the contrast between the two studies. Moreover, the study area is located towards the eastern/southern margin of the domal uplift proposed by von Eynatten et al. (2021) with likely less uplift/exhumation, as already emphasized in section 3.6. Given that the thermal anomalies mentioned are mainly local (as already stated by Freitag et al.) and an elevated heat flow of 80-85 mWm-2 still requires removal of 2.5-3 km of post-Early Triassic overburden (von Eynatten et al. 2021), I guess the contrast between the two studies remains within the uncertainties of the individual methods, implying that there is no need to call for increased heat flows or geothermal gradients.
Some parts appear over-referenced (and in this respect redundant in the Introduction and Methods parts, e.g. lines 125-126, 127-129, 133-134, 188-190). Please consider reducing to two or three major references as examples (e.g., …) or being more specific regarding information and respective references.
Minor points:
Line 80-82: sentence should be reformulated.
Line 97: the Cretaceous strata are even more related to the parallel structure further south, not labelled in figure 1 but abbreviated as ‘DF’ in the inset (Bayrischer Pfahl?, not explained in caption). This should be clarified for readers not familiar with the regional geology.
Line 275: it remains unclear whether these are 41 individual samples or 41 measurements on ca. 10 samples (please note that in the heading for table 1 and in the text (line 157) the numbers summing up to 41 (in case of GSC) are declared as measurements per sample). The same holds for line 222: 72 samples (or measurements per sample?) for bulk density and porosity. This should be consistent and clear for the readers without checking the Appendix.
Line 280: these terms should be used in figure 4a as well (i.e. avoid clayshale, mudshale, siltshale, they are rather unusual).
Line 308: quartz, pyrite, …
Line 325: … (2018) suggests vertical effective stresses … … and roughly equates to 700-2000 m true vertical depth.
Line 433: just for consistency, lower limit is 800 m in Fig.9, caption to Fig. 9 and in the text (line 450).
Line 464: not fully clear how the 1.1 km are deduced.
Line 484: von Eynatten et al. …
Line 631: though correct for German name rules, ‘von …’ is usually listed under ‘v’ in the reference lists of international journals. The same holds for ‘Le Bayon et al.’, etc. I guess.
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AC2: 'Reply on RC2', Simon Freitag, 12 May 2022
The authors thank the reviewer Prof. Dr. Hilmar von Eynatten for the constructive comments that helped in improving the content and quality of our manuscript. As recommended, we separated chapter 3 into ‘Results’ and ‘Discussion’, which contributed to a better structured and therefore more comprehensible manuscript. Reducing the amount of references in the over-referenced sections additionally increased the clarity of this manuscript. All the comments on the text have been addressed and reported in the table below.
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AC2: 'Reply on RC2', Simon Freitag, 12 May 2022
Simon Freitag et al.
Simon Freitag et al.
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