UNCONFORMITY.pptx

angular unconformity

1. UNCONFORMITY

2. Unconformity A surface of erosion or non-deposition separating younger strata from older rocks, along which there is evidence of subaerial erosional truncation (and, in some areas, correlative submarine erosion) or subaerial exposure, with a significant hiatus indicated.

3. Unconformity A surface or sequences of strata represent times of nondeposition and/or erosion separating younger strata from older rocks, along which there is evidence of subaerial erosional truncation or subaerial exposure, that encompass long periods of geologic time, perhaps millions or tens of millions of years ,with a significant hiatus indicated . INTRODUCTION

4. For 1 million years erosion occurred and removed 2 MY of rocks and giving rise to a 3 million year hiatus THE ORIGIN OF AN UNCONFORMITY The process of forming an unconformity deposition began 12 million years ago (MYA), continues until 4 MYA The last column is the actual stratigraphic record with an unconformity

5. Three types of surfaces can be unconformities: A disconformity is a surface separating younger from older rocks, both of which are parallel to one another A nonconformity is an erosional surface cut into metamorphic or intrusive rocks and covered by sedimentary rocks An angular unconformity is an erosional surface on tilted or folded strata over which younger rocks were deposited TYPES OF UNCONFORMITY

6. A disconformity between sedimentary rocks in California, with conglomerate deposited upon an erosion surface in the underlying rocks A Disconformity

7. An Angular Unconformity An angular unconformity, Santa Rosa

8. 8_9 (a) (b) (c) Layered sedimentary rocks Nonconformity Metamorphic rock Igneous intrusive rock Younger sedimentary rocks Angular unconformity Older, folded sedimentary rocks Disconformity Brachiopod (290 million years old) Trilobite (490 million years old)

9. A nonconformity in South Dakota separating Precambrian metamorphic rocks from the overlying Cambrian-aged Deadwood Formation A Nonconformity

10. Overlap: younger bed completely covers up and advances much beyond the limits of the underlying rocks. Can be seen during trangression. Offlap: it is reverse of overlap, lower beds of the upper series extends further than the younger ones due to marine regression.

11. Unconformities of regional extent may change from one type to another They may not represent the same amount of geologic time everywhere Types of Unconformities

12. Unconformities 3 types of unconformities are seen

13. Map Patterns of Unconformities Ig /Meta Rx Sed Rx Sed Rx Sed Rx Sed Rx Sed Rx

14. CRITERIA FOR RECOGNITION OF UNCONFORMITIES Unconformities may be recognized at the surface or on the geological map by: 1. Discordance of strike 2. Discordance of dip, without change of strike. The discordance may be by one amount or of direction. It should be made sure that the structure is not due to folding 3. Transgression of the outcrops . It is a clear test, provided such structure is not due to faulting. 4. Thinning out of beds (overlap) 5.Absence of beds in one area known to occur in neighbouring area

15. Recognition: Time interval Thickness of bed missing from the stratigraphic record Structural discordance Topographic irregularity Thinning out of beds (overlap) Presence of basal conglomerate