Using discontinuities for stratigraphic correlation

1. Using discontinuities for stratigraphic correlation EES 450 Dec. 12, 2013 Dr. Marta Clepper

2. Discontinuity surfaces • discontinuity – any interruption in sedimentation, usually a manifestation of nondeposition and erosion • used in place of unconformity to distinguish • stratigraphic break with no structural discordance with underlying beds • where there is no break in continuity of depositional sequence Photo by M. Clepper example of discontinuity surface

3. Discontinuity surfaces • show evidence of very significant physio-chemical changes • can be distinguished from simple bedding planes • they bear proof of breaks in sedimentation relevant to stratigraphic interpretation (Clari et al., 1995) Photo by M. Clepper example of discontinuity surface

4. Hardgrounds • surfaces of synsedimentary cemented carbonate layers exposed on the seafloor (Wilson and Palmer, 1992; McLaughlin et al., 2008) Photo by M. Clepper

5. Hardgrounds • simple hardgrounds typified by • minimal modification of surface topography with no evidence of boring or encrustation • preservation of encrusters – suggesting abrupt burial • little or no evidence for multiple generation of encrusters (McLaughlin et al., 2008) Photo by M. Clepper

6. Hardgrounds • composite hardgrounds • show evidence of multiple encrustation events • typically occur at sharp facies offsets • are traceable over broad areas (McLaughlin et al., 2008) Photo by M. Clepper

7. Authigenic mineral crusts • rich in iron (limonite), glauconite, and phosphate • authigenic mineral crusts have a lateral and stratigraphic distribution similar to composite hardgrounds • associated with condensation • upwelling brings cold, mineral-rich waters onto buildup • low-pressure, high temperature environment allows precipitation of minerals • cold waters restrict carbonate deposition

8. Discontinuity surfaces showing in situ crust of such mineral paragenesis indicate a considerable break in sedimentation, commonly in subtidal environments (Hillgärtner, 1998) Photo by M. Clepper

9. Any one exposure of the Lexington Limestone may exhibit a few tens to more than a hundred such surfaces, some of which may be mineralized, bored or encrusted. • Each of these • obviously represents a small break in the normal continuity of deposition, • and most are not correlatable beyond the exposure in which they occur. Photo by M. Clepper

10. However, a very few can be clearly recognized and correlated on a regional scale • these are the ones that have correlation value across the Lexington Limestone facies mosaic. • 3 prominent discontinuities divide Lexington Limestone

11. Three major discontinuities in the Lexington Limestone that have stratigraphic correlation value: • sub-Brannon discontinuity • shows no erosion • typically marked by prominent bored hardground • represents regional flooding surface OltB OltL Photo by M. Clepper sub-Brannon discontinuity between OltL and OltB – Frankfort 5 (F5) outcrop

12. Three major discontinuities in the Lexington Limestone that have stratigraphic correlation value: • sub-Sulphur Well discontinuity • pyritized, phosphatized corrosion surface • consists of 2-3 prominent hardgrounds within 2 ft of each other • can be traced westward into the Sebree Trough (Hohman, 1998) Olsw OltB Photo by M. Clepper sub-Sulphur Well discontinuity between OltB and Olsw – Frankfort 5 (F5) outcrop

13. sub-Sulphur Well discontinuity Olsw OltB

14. Photo by M. Clepper OltMB Three major discontinuities in the Lexington Limestone that have stratigraphic correlation value: • sub- Millersburg discontinuity • underlies 1st occurrence of Millersburg Member • flooding surface • locally has up to 1.5 ft (0.5 m) in relief • represents time of major transgression OltM sub-Millersburg discontinuity between OltM and OltMB – Interstate 64 1 (I-64 1) outcrop

15. Using Discontinuities in Correlations Correlation lines map showing the locations of the correlation lines created for the Lexington Limestone Formation using discontinuity surfaces

16. Using Discontinuities in Correlations Correlation line A-A’ showing correlation across central Bluegrass region using the sub-Sulphur Well discontinuity as a datum

17. Using Discontinuities in Correlations At this outcrop (F6) the sub-Sulphur Well discontinuity is situated between OltB and OltM

18. Using Discontinuities in Correlations • How can you know that this is the sub-Sulphur Well discontinuity? • 3 composite hardgrounds within 1 ft of each other • Different tongues of the Olt determined by lithology and bedding OltM OltB

19. Using Discontinuities in Correlations At this outcrop (F2) the sub-Sulphur Well discontinuity is situated between Olb and OltM

20. Using Discontinuities in Correlations Correlation line A-A’ showing correlation across central Bluegrass region using the sub-Sulphur Well discontinuity as a datum

21. CV F2 F6

22. Conclusions • With careful analysis, discontinuity surfaces can be identified within a stratigraphic section and if they can be traced over large areas, they can prove to be a useful tool in correlation of stratigraphic units. • In the case of the Lexington Limestone, features that helped to identify these surfaces were • approximate stratigraphic position • the nature of the surface(s) • extent of erosion • While discontinuity surfaces alone are not enough to fully interpret complex units like the Lexington Limestone, using them in concert with other data, such as facies identification and isotope analysis can be very beneficial.