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AUTOGENIC PROGRADATION of BAYHEAD DELTAS DURING SEA- LEVEL RISE WITHIN INCISED VALLEYS: THEORY, EXPERIMENT and FIELD EXAMPLES L. Guerit1, B.Z

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AUTOGENIC PROGRADATION of BAYHEAD DELTAS DURING SEA- LEVEL RISE WITHIN INCISED VALLEYS: THEORY, EXPERIMENT and FIELD EXAMPLES L. Guerit1, B.Z AUTOGENIC PROGRADATION OF BAYHEAD DELTAS DURING SEA- LEVEL RISE WITHIN INCISED VALLEYS: THEORY, EXPERIMENT AND FIELD EXAMPLES L. Guerit1, B.Z. Foreman2, C. Chen3, C. Paola4, S. Castelltort3 1University of Rennes, France, 2Western Washington University, USA, 3University of Geneva, Switzerland, 4University of Minnesota, USA A CAN BAYHEAD DELTAS BE AUTOGENIC ? V2 hs Lv H WE PROPOSE A THEORETICAL h bayhead diastems W highstand delta FRAMEWORK THAT PREDICTS β V1 B P ~50 km offshore Brazos Incised Valley AUTOGENIC PROGRADATION α θ P SW NE Cretaceous 0.02 ace fine-grained marine & transgressive deltaic sediments surf R bedrock marine sea bedding t oxbow lake deposits 0.04 oo le P valley apex B um /in axim ier m 0.06 arr Feeder Cenozoic b estuarine R x-bedded sands bedrock Trinity 0.08 seismicR line Fluvial fan Lv trans. delta 0.10 Brazos River Valley River P 0.12 lowstand delta sequenchannelce lagbounda & sands ry / valley thalw2eg km β hs bayhead WE DEVELOP EXPERIMENTAL MODELS TO VALIDATEV2 THE Valley Valley bottom deltas (sec) Time Travel Way Two C Galveston Bay (Trinity Valley) V1 0 MODEL: AUTO-ADVANCE DO OCCUR h modern SW A Guerit et al, Geology, 2020NE Valley basement B α bayhead stage 3 -10 BeaumontValley length 2.05 m Galveston stage 2 delta Fm 3 basin mud stage 3 C Bay -20 Sediment width 0.8 m Valley h =h Houston flood-tidal delta high auto-advance R middlefeeder Deweyville terrace ~50 km offshore Brazos Incised Valley onset of NortH (m) Depth -30 stage 2 B A bayhead delta Deweyville terrace onset of back-stepping SW 5 km fine-grained marine & transgressive deltaic sediments NE 2 auto- Cretaceous fluvial stage 1 0.02 100 km -40 WE APPLY OUR deposition in the valley Stage 2 advance Colorado- bedrock sea bed oxbow lake deposits A/S 0.04 Time (min) (A>S) Matagorda Brazos D FRAMEWORKGalveston Bay Dip Section TO (Trinity THE Valley) Bay Delta offshore N 0.06 Water panel c section S 1 incised Cenozoic-2 stage 3 TRINITY AND BRAZOS x-bedded sands Stage 1 Stage 3 valleys supply Trinity 0.08 (A<S) (A<S) bedrock-6 stage 2 Time (min) stage 3 seismic line RIVER SYSTEMS 90 95 100 105 110 115 -10 0.10 120 130 1400 150 160 trans. delta 90 100 110 0 0.2 0.4 0.6 0.8 1 Brazos River Valley (TEXAS): FIELD Distance from feeder (cm) Gulf of Mexico -14 River 0.12 channel lag & sands 2 km Normalized base level Valley bayhead -18 OBSERVATIONS ARE A stage 2 deltas (sec) Time Travel Way Two B Depth (m) Depth C Galveston Bay (Trinity Valley) -22 WELL0 EXPLAINED BY modern SW ValleyNE length 2.05 m -26 bayhead -10 OUR MODEL 10 km stage 3 stage 1 stage 2 Beaumont -30 delta Galveston Sediment Fm width 0.8 m Valley Bay -20 basin mud stage 3 feeder auto-advance Houston flood-tidal delta high 7 E Brazos Valley 7 F Trinity Valley middle Deweyville terrace NortH (m) Depth stage 2 Accommodation -30 bayhead delta Deweyville terrace Accommodation 5 km 6 Sediment supply 6 fluvial Sediment supply -40 stage 1 /yr) 100 km /yr) 3 5Colorado- 3 5 m m stage 3: auto-advance Time (min) 6 6 D Galveston Bay Dip Section (Trinity Valley) Matagorda 4 Brazos 4 Bay Delta offshore N panel c section S 3 incised 3 -2 stage 3 Water valleys Rate (10 Rate 2 (10 Rate 2 -6 stage 2 supply stage 3 1 1 -10 Time (min) 5 10 15 20 25 5 10 15 20 25 90 95 100 105 110 115 0 Gulf of Mexico 0 120 130 140 150 160 0 20 40 60 80 100 0-14 20 40 60 80 100 90 100 110 Base level (m) Time (ky) Base level (m) Time (ky) Distance from feeder (cm) -18 stage 2 Depth (m) Depth -22 -26 10 km stage 1 -30 7 E Brazos Valley 7 F Trinity Valley Accommodation Accommodation 6 Sediment supply 6 Sediment supply /yr) /yr) 3 5 3 5 m m stage 3: auto-advance 6 4 6 4 3 3 Rate (10 Rate 2 (10 Rate 2 1 1 5 10 15 20 25 5 10 15 20 25 0 0 0 20 40 60 80 100 0 20 40 60 80 100 Base level (m) Time (ky) CAN BAYHEAD DELTAS BE AUTOGENIC ? This materiel is published (Guerit et al, Geology, 2020) highstand delta bayhead diastems P P See next slides for focus on: face g sur R marine odin t theoretical model m o nle P valley apex ximu r/i • ma rrie ba estuarine R • physical model R field application P lowstand delta sequence boundary / valley thalweg • MOTIVATIONS IMPLICATIONS A growing list of observations suggest that autogenic Bayhead deltas and their paired flood-tidal and barrier complexes dynamics is able to produce large-scale stratigraphic may result from autogenic events related to the interplay of patterns similar to the ones associated with allogenic accommodation and sediment supply. factors such as sea-level or sediment flux changes. The relevant parameters for development of bayhead deltas can Here, we focus on incised valley fills, where out of be estimated for paleocase studies and provide additional sequence bayhead deltas are often observed and constraints on sequence stratigraphic reservoir models. attributed to external factors. However, some authors propose that they could be autogenic. Our finding provides more evidence that geometry and mass- balance interactions play a major role in dictating large-scale We build on this idea and show that bayhead deltas can stratigraphic patterns and overall sensitivities of sediment transport result from the interplay between sediment supply and the systems. These phenomena appear ubiquitous enough to warrant evolving geometry of incised valleys during steady base- consideration of autogenic controls on stratigraphy at the outset of level rise to create an autogenic stratigraphic pattern that any stratal analysis. we term auto-advance MAIN REFERENCES • Aschoff et al (2018) Recognition and significance of bayhead delta deposits • Tomer et al (2011) Autogenic hiatus in fluviodeltaic successions: in the rock record: A comparison of modern and ancient systems, Sed. Geometrical modeling and physical experiments, JSR • Hajek and Straub (2017) Autogenic sedimentation in clastic stratigraphy, • Trower et al (2018) Erosional surfaces in the Upper Cretaceous Castlegate AREPS Sandstone (Utah, USA): Sequence boundaries or autogenic scour from • Muto and Steel (1992) Retreat of the front in a prograding delta, Geol. backwater hydrodynamics?, Geol. • Simms and Rodriguez (2015) The influence of valley morphology on the • Zaitlin et al (1994) The stratigraphic organization of incised-valley systems rate of bayhead delta progradation, JSR associated with relative sea-level change, SEPM WE PROPOSE A THEORETICAL FRAMEWORK THAT Under constant S and R, three distinct A/S stages occur during the PREDICTS AUTOGENIC PROGRADATION inundation of the valley. A • Stage 1: A increases (V1) but remains smaller than S (A < S). This induces a progradational regime in the lower and distal part of the V2 hs Lv valley. A H h Stage 2: A increases and then decreases as the base-level rises above V2 Lv • hs W the shelf edge due to the change in geometry (V2), but A is always larger β than S. Deposition within the valley is thus retrogradational. H V1 h α θ Stage 3: A becomes smaller than S, and progradation resumes despite W • β the overall context of relative sea-level rise. This is what we term “auto- V1 B α θ advance”. This progradation is short-lived with respect to the whole Feeder filling sequence as the continuing sea-level rise eventually floods the system and induces marine deposition. B Fluvial fan Lv Feeder hs V2 β Fluvial fan Valley bottomLv V1 h Valley basement hs V2 αβ Valley bottom C 3 V1 h Valley basement hR=h αonset of onset of back-stepping auto- C 3 2 deposition in the valleyh =h Stage 2 advance While sea level remainsR below the edge of the A/S (A>S) onset of incised valley (hR < h), the volume in the valley is: 2 1 onset of back-stepping auto- Stage 1 Stage 3 deposition in the valley 3Stage 2 advance (A<S) hR (A<S) A/S V1 = (A>S) 1 0 3αβ 0 Stage 10.2 0.4 0.6 0.8 Stage 31 (A<S) Normalized base level (A<S) When sea level rises above the edge of the This simple model predicts a period of progradation (auto-advance) in 0 the late stage of valley inundation, as a consequence of the prismatic 0 valley0.2 (hR > h0.4), the volume0.6 changes0.8 to: 1 valley geometry. Normalized base level The stratigraphic signature of auto-advance is similar to the one W hR − h hR hR − h V2 = Lv − − (hs + h − hR) that would result, for example, from a transient increase in 6Lv ( β ) ( α β ) sediment supply. movie available here: https://doi.org/10.1130/GEOL.S.13046516.v1 WE DEVELOP EXPERIMENTAL MODELS TO VALIDATE THE MODEL: AUTO-ADVANCE DO OCCUR A B Valley length 2.05 m Sediment width 0.8 m Valley feeder auto-advance Time (min) Water supply Time (min) 90 95 100 105 110 115 120 130 140 150 160 90 100 110 Distance from feeder (cm) A B Valley length 2.05 m Sediment width 0.8 m Valley feeder auto-advance Time (min) Water supply Time (min) 90 95 100 105 110 115 120 130 140 150 160 90 100 110 Distance from feeder (cm) • a nonerodible V-shape valley with a slope α of 0.06 inserted within a 5 × 5 × 0.6 m tank.
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