DETERMINING SEQUENCE OF EVENTS FROM A GEOLOGIC CROSS SECTION A geologic cross section is a vertical view of strata as they would appear in a road cut or quarry face. By interpreting the sequence of events which produced the strata and structures displayed in a cross section, it is possible to gain an insight into the geologic history of a region. The sequence of events portrayed in a geologic cross section can be deciphered using the following five rules: Rule 1. Superposition - Unless strata have been overturned by extreme folding, the oldest layer will be on the bottom and the layers become progressively younger above. Rule 2. Folding, Faulting and Intrusions - Strata and unconformities must have been formed before they could be folded, faulted or cut by intrusions and are therefore older than the period of folding, faulting or intrusion. Rule 3. Unconformities - An erosional surface (unconformity) is always younger than the strata upon which it is developed and the features that it truncates, but it is older than overlying strata or features which cut across it.. Rule 4. Cross-cutting Relationships - A fault or intrusion that offsets or cuts across another fault or intrusion is always the younger because the original fault or intrusion had to exist before it could be offset by a fault or cut by an intrusion. Rule 5. Folding Precedes Faulting - In general, when placed under stress, rocks fold as much as possible before they rupture and fault. SEQUENCE OF EVENTS IN A SECTION OF THE GRAND CANYON H D K 4 O Y R Unconformity Z W T A N 3 J Unconformity S P E C 2 U X F 1 B Unconformity L The above diagram represents a geologic cross section from the Grand Canyon. The following is a stepby-step explanation of how the sequence of events for this section was deciphered. Unconformities represent breaks in the depositional history for a region. They naturally divide complex sections into more easily understood units. One should always begin by dividing the cross section into units bounded by unconformities and then determine the sequence of events for each unit separately employing the rules stated above. Using the unconformities, this cross section can be divided into four units, labeled 1-4. Based upon superposition (Rule 1), the sequence of units from oldest to youngest is 1, 2, 3 and 4. Unit 1 is bounded above by unconformity L and part of unconformity S. Unconformity L is truncated by unconformity S and therefore must be older than S (Rule 3). Unit 1 contains strata X, B and part of C. Because C cuts across unconformity L, C is younger than L (Rules 2 and 3). The remaining units, X and B, are truncated by the unconformities and therefore must predate them. (Rule 3). B is an intrusion into X and therefore could only form after X (Rule 2). For unit 1 the sequence of events therefore has to be: 1. Formation of X 2. Intrusion B 3. Unconformity L Intrusion C and unconformity S occurred after Unit 1. Unit 2 is bounded below by unconformity L and above by unconformity S. It contains the rock layers F, U, E and part of intrusion C. Because intrusion C extends across unconformity S, it must be younger than the unconformity (Rules 2 and 3) Unit 2 rests directly upon unconformity L and therefore must be younger than it (Rule 3). Based upon super position (Rule 1) layers F, U and E were deposited in that order. Sedimentary strata are normally horizontal when deposited, but layers F, U and E are steeply inclined. This means they were folded after they were deposited (Rule 2). Because the folds are cut by unconformity S, which is not folded, folding occurred after deposition of F, U and E, but before unconformity S (Rule 2). For unit 2, the sequence of events therefore has to be: 4. Deposition of F, U and E, in that order 5. Folding of unit 1 and F, U and E 6. Unconformity S Intrusion C occurred after Unit 2. Unit 3 is bounded by unconformity S below and unconformity Z above. It includes layers P, J, N, A, T, W and part of C. Layer P rests directly upon unconformity S and therefore must be younger than unconformity S (Rule 3). Based upon superposition (Rule 1), the other layers must have been deposited in the following order from oldest to youngest: J, N, A, T and W. Intrusion C extends into, but not beyond layer P. C could have been intruded into the section at any time following the deposition of layer P and before exposure of the geologic section by present-day erosion. For unit 3, the sequence of events has to be: 7. Deposition of P. J, N, A, T and W, in that order - any time after deposition of P, intrusion C may have occurred 8. Unconformity Z Unit 4 is bounded by unconformity Z and the present-day erosional surface. It contains the layers Y, R, O, K, H and D. Layer Y rests directly upon unconformity Z and therefore must be younger than unconformity Z (Rule 3). Based upon superposition (Rule 1), the other layers must have been deposited in the following order from oldest to youngest Y, R, O, K, H and D. Sometime after the deposition of layer D, erosion reduced the land to its present configuration For unit 4, the sequence of events would be: 9. Deposition of Y, R, O, K, H and D, in that order 10. Erosion to the present-day surface. The following two page contain four sequence-of-events problems. They can be solved using the same methods and rules employed above. For each of the problems number the listed events in the correct order beginning with 1 as the first (oldest) event and proceed to the youngest. Answer the associated questions. Answers are provided at the end of the exercises. Unconformity A Problem 1. O N M L H I Fault Y G J H F E Z X Unconformity C Problem 1. a. b. c. d. e. f. g. h. i. j. k. _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ Problem 2 a. b. c. d. e. f. g. Deposition of L and M Deposition of N and O Deposition of E, F and G Deposition of H, I and J Intrusion of granite X Intrusion of basalt Z Unconformity A Type _____________ Unconformity C Type _____________ Erosion to present surface Fault Y Type ______________ Folding of X through J Type h. _____ J Problem 2 _____ _____ _____ _____ _____ _____ _____ Deposition of A, E, I, O, U, and Y Deposition of S and V Deposition of W, Z, R, N, J and D Unconformity L Type__________________ Erosion to present surface Intrusion of basalt C Folding of beds A through V. Types ______________________ Fault P Type___________________ D D J N R N Z R Unconformity L W Z Y S U V E S O U I W I O Y O E C U A Fault P Unconformity O Fault M Problem 3 W U O J F L V D C H U V Z K D A Y E Y Y I G S T T Problem 3 _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ N Problem 4 Deposition of K and W Deposition of N, S T and I Deposition of E, G, R, A H and C Deposition of D, Z and V Deposition of L, O, U, F and J Folding of layers N through C Unconformity B Type ___________________ Unconformity X Type___________________ Unconformity O Type___________________ Intrusion of Y Fault M Type_______________________ Erosion to present surface a. b. c. d. e. f. g. h. i. j. _____ _____ _____ _____ _____ _____ _____ _____ _____ _____ Deposition of A Through Y Deposition of C. F and B Deposition of T and I Unconformity S Type___________________ Unconformity Q Type___________________ Intrusion of W Fault P Type_______________________ Fault J Type_______________________ Erosion to present surface Folding Type _______________________ Fault P Problem 4 Unconformity Q F Unconformity S Unconformity Q I T F Y Z C X G H E K D D M M A G X Z U U L L D K W H Fault P C B B F A Unconformity B Z Y Y R a. b. c. d. e. f. g. h. i. j. k. l. Unconformity X G M Fault J Correlation Below are three illustrated rock exposures. Correlate between them using lithostratigraphy, biostratigraphy and chronostratigraphy. Period Fossils A B C D E F G H I J K L M N O P Q R S T U V W X Y Z 1 2 3 4 5 6 7 Zone Silurian Zone E Zone D Ordo. Zone C Zone B Camb. Zone A No Fossils No Fossils F, G, L, N No Fossils No Fossils K, N, F, F, G, K, L, R No Fossils 3, 2, X, R, N K, F, N A, C, I, 3, 4 A, 3, C, I H, K, P, Q G, W, 2, X A, C, G, O, I, L, O, S A, C, E, O, K, M A, D, E H, M, G A, C, I, K E, Q, O A, C, E, K, O E, J, A, K J, S, V, 1, 2, 3 A, J, M, B A, M, P, Q, 7 J. M. B, P, T B, Z, 2, 3 No Fossils No Fossils 3, P, A, B, V No Fossils No Fossils Problem 1 Problem 2 a. 09 b. 10 c. 04 d. 05 e. 01 f. 02 g. 08 - angular unconformity h. 03 - nonfonformity i. 11 j. 07 - reverse fault k. 06 - anticline a. b. c. d. e. f. g. h. Problem 3 a. 10 b. 01 c. 02 d. 05 e. 07 f. 03 g. 04 - angular unconformity h. 06 - disconformity i. 08 - disconformity j. Intrusion Y occurred after the deposition of Z (post 05) k. 09 - normal fault l. 11 01 02 05 04 - angular unconformtiy 08 07 03 - syncline, anticline, syncline 06 - normal fault Problem 4 a. 01 b. 04 c. 07 d. 06 = disconformity e. 03 - angular unconformity f. 09 or 10 g. 05 - normal fault h. 08 - reverse fault i. 9 or 10 j. 02 - anticline, syncline E E E E SILURIAN D E D D D C C B D C D ORDOVICIAN C C B B B A A A CAMBRIAN A