Sequence Stratigraphic Analysis of Mixed, Reefal Carbonate and Siliciclastic Systems. Ashton Embry. Geological Survey of Canada.

Transcription

1 Sequence Stratigraphic Analysis of Mixed, Reefal Carbonate and Siliciclastic Systems Ashton Embry Geological Survey of Canada Outline Sequence Stratigraphy Sequence Surfaces and Sequence Boundaries for Siliciclastics Sequence Surfaces and Sequence Boundaries for Carbonates/Reefs Sequence Surfaces and Sequence Boundaries for Mixed Reef and Siliciclastic Successions Correlating a Reef/Siliciclastic Succession 1

2 REEF SILICICLASTICS How do we use sequence stratigraphy to correlate from a reef to off-reef siliciclastics? Where is the sequence boundary? Importance of Doing Sequence Stratigraphy Properly Sequence stratigraphy provides an excellent methodology for: Correlating a stratigraphic succession. Subdividing it into genetic, quasichronostratigraphic units for facies analysis and paleogeographic reconstruction. 2

3 SEQUENCE STRATIGRAPHY Consists of : The recognition and correlation of stratigraphic surfaces which represent depositional breaks or changes in depositional trend in the rock record. SEQUENCE STRATIGRAPHY Sequence surfaces are recognized by sedimentological criteria and geometrical relationships between the surface and strata above and below the surface. 3

4 SEQUENCE STRATIGRAPHY Consists of : The description and interpretation of resulting, genetic stratigraphic units bound by those sequence surfaces. USING SEQUENCE STRATIGRAPHY 1) For constructing an approximate chronostratigraphic, correlation framework to facilitate facies analysis and related endeavours. 4

5 USING SEQUENCE STRATIGRAPHY 2) For bounding individual sequence stratigraphic units for mapping and communication purposes. USING SEQUENCE STRATIGRAPHY 3) For allowing the sedimentary record to be interpreted in terms of base level changes. 5

6 USING SEQUENCE STRATIGRAPHY 4) In conjunction with facies analysis, for providing a framework and theoretical basis for interpreting depositional history and paleogeographic evolution. The Five Material-Based Surfaces of Sequence Stratigraphy Unconformities (time barriers) or low diachroneity surfaces (approx. time surfaces) 1) Subaerial Unconformity (SU) 2) Shoreline Ravinement (SR) 3) Maximum Regressive Surface (MRS) 4) Maximum Flooding Surface (MFS) 5) Slope Onlap Surface (SOS) 6

7 The Five Material-Based Surfaces of Sequence Stratigraphy Unconformities (time barriers) or low diachroneity surfaces (approx. time surfaces) 1) Subaerial Unconformity (SU) 2) Shoreline Ravinement (SR) 3) Maximum Regressive Surface (MRS) 4) Maximum Flooding Surface (MFS) 5) Slope Onlap Surface (SOS) Slope Onlap Surface An unconformable stratigraphic surface which is developed in marine slope strata and onto which there is demonstrable onlap by marine strata. 7

8 SOS forms in siliciclastic successions when the shoreline reaches the shelf edge and much of the slope becomes starved. SOS Posamentier and Vail, 1988 SOS is onlapped by slope deposits during both base level fall and base level rise. Posamentier and Vail,

9 SR-U Slope Onlap Surface (SOS) Quaternary Strata, Gulf of Mexico Seismic from Posamentier, 2003 SOS Grammer et al, 2005 When the shelf is exposed during base level fall, carbonate sedimentation essentially ceases except for rock falls and localized carbonate production which onlap the starved slope (SOS). 9

10 Inductive Model for Material-Based Start Regression Start Transgression T Base Level Fall Maximum Flooding Surface Maximum Regressive Surface Slope Onlap Surface Shoreline Ravinement Subaerial Unconformity Embry, 1995 Sequence Stratigraphic Surfaces in Siliciclastics 10

11 Early Base Level Fall Mid Base Level Rise End Base Level Fall Early Base Level Fall Sequence Stratigraphic Surfaces in Carbonates 11

12 SR-U In most cases, the strata directly overlying an unconformity in carbonate strata are marine in origin. The is indicates that a previously formed subaerial unconformity (SU) was modified during transgression. In such cases, the surface is best referred to as an Unconformable Shoreline Ravinement (SR-U). Fairholme Gp (Late Devonian), Rocky Mtns, Slope Onlap Surfaces Whalen et al.,

13 Grammer et al, 2005 Sequence Stratigraphic Surfaces Bahamas Bank Sequence Stratigraphic Surfaces Carbonate Platform/Slope/Basin 13

14 Sequence Stratigraphic Surfaces Reefal Carbonate Wheeler Diagram Sequence Stratigraphic Surfaces Reefal Carbonate 14

15 Next Question Where s the Sequence Boundary Depositional Sequence A depositional sequence is a stratigraphic unit bound by subaerial unconformities and their correlative surfaces (Van Wagoner et al, 1988). 15

16 Depositional Sequence Boundaries (Subaerial unconformities/shoreline ravinements on the basin flank and correlative surfaces basinward ) SU/SR-U SU/SR-U Correlative Surfaces Correlative Surfaces Correlative surfaces are sequence stratigraphic surfaces which join with the end of the SU/SR-U and with each other, so as to form a single, continuous, through going boundary. SU/SR-U Correlative Surfaces 16

17 SU/SR-U Correlative Surfaces Given that: ` 1) The correlative surfaces must adjoin to the basinward termination of the SU. 2) The Subaerial Unconformity reaches its maximum basinward extent at the end of base level fall (start base level rise). The correlative surfaces must develop at, or soon after, the start of base level rise to be able to join with the end of the SU/SR-U. Developed at or soon after start Rise Developed well after Rise The marginal unconformity (SU/SR-U) and the correlative surfaces must adjoin so as to form one continuous boundary. 17

18 Depositional Sequence Boundary Placement Due to the requirement that the correlative surfaces portion of the SB develop at and soon after the start of base level rise: All sediments deposited during the previous base level fall must lie below the depositional sequence boundary. Schlager, 2005 Over the past 30 years many workers have put the sediments deposited during base level fall on top of the sequence boundary. 18

19 SU/SR-U Revised The Depositional Sequence Boundary must be placed ABOVE the truncated strata which were deposited during base level fall. Attached Attached Debris Debris SB Detached Debris debris Handford and Loucks, 1993 This SB is unacceptable because it places sediments deposited during base level fall (orange) above the SB. 19

20 SU/SR-U Attached Debris X SB Detached debris SB Revised Sediments Deposited During Base Level Fall (orange) Are Placed Below the SB Anselmetti et al.,

21 Sequence Stratigraphic Surfaces Carbonate Platform/Slope/Basin Depositional Sequence Boundary Placement Sequence Stratigraphic Surfaces Reefal Carbonate Depositional Sequence Boundary Placement 21

22 Wheeler Diagram Depositional Sequence Boundary Placement Sequence Stratigraphic Surfaces in Mixed Reefal Carbonates and Siliciclastics 22

23 23

24 Sequence Stratigraphic Model Mixed Reefal Carbonates and Siliciclastics 24

25 SR-U Ladinian Carbonate Platform, Lombardy Basin, Italy Berra, 2007 SU/SR-U Berra,

26 Berra, 2007 SR-U MFS MRS Ladinian Carbonate Platform, Lombardy Basin, Italy Berra,

27 Kendall, pers.comm., 2008 Permian Basin Depositional Sequence Boundaries in the Brushy Canyon Formation of the Permian Basin are usually placed at the base of deep water sandstone units (red lines) which are interpreted to have been deposited during base level fall. Kendall, pers.comm., 2008 Permian Basin Revised Depositional Sequence Boundaries (red) are best placed near or at the top of sandstone units 27

28 Depositional Sequence Boundaries (red) are best placed near or at the top of sandstone units rather than at the base of them. Carbonates and Basinal Siliciclastic Turbidites Sarg, 1988 Depositional Sequence Boundaries Were Inappropriately Placed Below the Deep Water Sandstones Deposited During Base Level Fall 28

29 Carbonates and Basinal Siliciclastic Turbidites Sarg, 1988 The Depositional Sequence Boundary is most reasonably placed on top of the deep water clastics rather than at the base. Depositional Sequence Boundary Carbonate Platform/Slope and Deep Water Siliciclastics 29

30 SOS Fairholme Gp (Late Devonian), Rocky Mtns, Slope Onlap Surfaces and Sequence Boundaries Whalen et al.,

31 Correlating from Reef to Siliciclastics with Sequence Stratigraphy SR-U MFS MRS Correlating from Reef to Siliciclastics with Sequence Stratigraphy 31

32 Subsurface Correlation from Reef to Siliciclastics with Sequence Stratigraphy Sequence Stratigraphic Surfaces Reefal Carbonate and Adjacent Siliciclastics 32