AGU 2014, San Francisco

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AGU 2014, San Francisco The applicability of detrital zircon in determining depositional ages and sedimentation rates in a retroarc foreland basin, Upper Cretaceous Magallanes-Austral basin,

1 AGU 2014, San Francisco The applicability of detrital zircon in determining depositional ages and sedimentation rates in a retroarc foreland basin, Upper Cretaceous Magallanes-Austral basin, Patagonia Theresa M. Schwartz 1, Marcelo Leppe 2, Stephan A. Graham 1, &Tess A. Menotti 1, 3 1 Stanford University 2 Instituto Antártico Chileno, Punta Arenas, Chile 3 Chevron Corporation, Houston, TX 1

2 Introduction Study Area The Magallanes-Austral basin (MAB) Retroarc foreland basin Confined by south Andean arc and the Rio Chico-Dungeness Arch Highly asymmetric basin profile Deep-marine foreland trough hugging the fold-thrust belt; contains up to ~7 km of sediment Campanian-Maastrichtian Dorotea Fm shallow-marine cap to deepmarine foredeep succession Exposed in frontal monocline of modern fold-thrust belt (Schwartz & Graham, 2014) 2

3 Introduction Basin Architecture 3

4 Introduction Basin Architecture (modified from Fosdick et al., 2014) 4

5 Introduction Basin Architecture (modified from Fosdick et al., 2014) 5

After that, buried by up to 5 km of overburden which was subsequently stripped

6 Motivation Relatively poor constraint on final filling of the MAB Linked slope and shelf-edge delta existed at this latitude Strongly progradational system After that, buried by up to 5 km of overburden which was subsequently stripped off Continued marginal marine deposition by mid-eocene time (Fosdick et al., 2014) 6

assemblages (reptiles, invertebrates, plants) Very

7 Motivation Constraints on absolute ages of shallow marine units are few Highly diverse fossil assemblages (reptiles, invertebrates, plants) Very poor preservation of ashes What can detrital zircon tell us? 7

8 Fundamental Assumptions 1. The south Andean arc was continually active (extruding volcanogenic rock/sediment) throughout the Upper Cretaceous Volcanic carapace not preserved Maast.-Camp. batholith rocks exist along the length of the arc 8

9 Fundamental Assumptions 1. The south Andean arc was continually active (extruding volcanogenic rock/sediment) throughout the Upper Cretaceous Volcanic carapace not preserved Maast.-Camp. batholith rocks exist along the length of the arc 2. The arc and northern Magallanes embayment were fluvially interconnected Long-lived, orogen-parallel & basin-axial paleodispersal system Long-term derivation of sediment from the arc Evidence for large river systems feeding the basin (Dorotea Fm) 9

10 Fundamental Assumptions 1. The south Andean arc was continually active (extruding volcanogenic rock/sediment) throughout the Upper Cretaceous Volcanic carapace not preserved Maast.-Camp. batholith rocks exist along the length of the arc 2. The arc and northern Magallanes embayment were fluvially interconnected Long-lived, orogen-parallel & basin-axial paleodispersal system Long-term derivation of sediment from the arc Evidence for large river systems feeding the basin (Dorotea Fm) 3. Volcanic detritus was not sequestered long-term in the arc or fold-thrust belt No preserved intra-arc or piggyback basins Coupled DZ and ash data indicate insignificant lag times between extrusion of zircon and deposition in the foredeep 10

11 Detrital Zircon Data Sample Distribution 11

12 Detrital Zircon Data Sample Distribution 12

13 Detrital Zircon Data Northern Ultima Esperanza 13

14 Detrital Zircon Data Northern Ultima Esperanza 14

15 Detrital Zircon Data Northern Ultima Esperanza 15

16 Detrital Zircon Data Northern Ultima Esperanza 16

17 Detrital Zircon Data Northern Ultima Esperanza 17

18 Detrital Zircon Data Northern Ultima Esperanza 18

19 Detrital Zircon Data Northern Ultima Esperanza 19

20 Detrital Zircon Data Northern Ultima Esperanza 20

21 Detrital Zircon Data Northern Ultima Esperanza 21

22 Detrital Zircon Data Northern Ultima Esperanza 22

23 Detrital Zircon Data: Maximum depositional age (MDA) An example: Use youngest age group present in the sample Calculate weighted mean age 23

24 Detrital Zircon Data: Maximum depositional age (MDA) An example: Use youngest age group present in the sample Calculate weighted mean age 24

25 Detrital Zircon Data: Maximum depositional age (MDA) 25

26 Deltaic Sedimentation Rates Deltaic aggradation (or, subsidence) 1D subsidence model (PetroMod ) => decompacted sedimentation rates Includes basement terranes and entire basin fill For Dorotea Fm, ages of FAs inferred from DZ and fossil data Deltaic progradation Similar thermal history in North and South (Fosdick et al., 2014) Simple calculation of R=D/t 26

27 Deltaic Sedimentation Rates 27

28 Deltaic Sedimentation Rates Deep-marine stage: 0.7 mm/year Shallow-marine stage: 0.2 mm/year 28

29 Deltaic Sedimentation Rates Deep-marine stage: 0.7 mm/year Shallow-marine stage: 0.2 mm/year By comparison Delta progradation rates = 6 to 14 mm/yr 29

30 Deltaic Sedimentation Rates 30

31 Summary 1. Suites of detrital zircon data yield MDAs that are potential TDAs in the MAB Paleodrainage network continuously routed young zircon to the marine foreland basin 2. Sedimentation rates derived from TDAs indicate: Subsidence rates slowed in latest Cretaceous Aggradation outpaced by southward progradation Rates consistent with those of other advancing margins 3. Tectonic implications & remaining questions Progradation potentially forced by coeval thrust-front advance, indicating filling and over-spilling of MAB Thermochronology indicates another 5 km of sedimentary burial Still much to figure out! 31

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