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 AUTOGENIC PROGRADATION α θ P SW NE Cretaceous 0.02 ce fine-grained marine & transgressive deltaic surfa R bedrock sea beding deposits marine0.04 ood let 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 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 Two Way Travel Time (sec) C Galveston Bay (Trinity Valley) V1 0 MODEL: AUTO-ADVANCE DO OCCUR h modern SW A Guerit et al, , 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 Valley width 0.8 m h =h Houston -tidal delta high auto-advance R middlefeeder Deweyville terrace ~50 km offshore Brazos Incised Valley onset of

North Depth (m) -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 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

North Depth (m) 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 2 Rate (10 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) -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 2 Rate (10 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 •

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 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 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 (AS) 1 0 3αβ 0 Stage 10.2 0.4 0.6 0.8 Stage 31 (A 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 Valley width 0.8 m 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 Valley width 0.8 m 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. • constant water and sediment provided by a computer- controlled feeder, • 50:50 mixture of quartz (white) and anthracite coal (black) grains to simulate the coarse and fine fractions of the sediment load, respectively. • constant base-level rise achieved by raising a computer-controlled . Base level in the tank set at the base of the valley outlet at the beginning of the experiment. It ended once the entire fan-valley system was flooded (total run time:130 min). • position of the coarse-grained delta front from orthorectified images taken every minute ~50 km offshore Brazos Incised Valley WE APPLYA OUR FRAMEWORK TO THE B Cretaceous 0.02 SW fine-grained marine & transgressive deltaic sediments NE TRINITY AND BRAZOS RIVER SYSTEMS oxbow lake deposits (TEXAS):bedrock FIELD OBSERVATIONS ARE 0.04 sea bed WELL EXPLAINED BY OUR MODEL 0.06 Cenozoic x-bedded sands A bedrock Trinity B 0.08 ~50 km offshore Brazos Incised Valley seismic lineSW NE Cretaceous 0.02 0.10 fine-grained marine & transgressive deltaic sediments trans. delta oxbow lake deposits bedrockBrazos River Valley 0.04 sea bed River 0.12 channel lag & sands 2 km bayhead0.06 Cenozoic Valley x-bedded sands bedrock Trinity deltas0.08 Two Way Travel Time (sec) Galveston Bay (Trinity Valley) seismic line C 0 SW NE moderntrans. delta 0.10 Brazos River Valley River bayhead 0.12 channel lag & sands stage 3 2 km bayhead -10 Beaumont Valley Galveston stage 2 deltadeltas Two Way Travel Time (sec) Galveston Bay (Trinity Valley) Fm C basin mud stage 3 Bay0 -20 NE modernHouston SW flood-tidal delta high bayhead stage 3 middle Deweyville terrace -10 Beaumont North Depth (m) -30 bayheadstage 2 delta stage 2 Deweyville terrace delta Galveston Fm basin mud stage 3 5 km Houston Bay -20 fluvial stage 1 high 100 km flood-tidal-40 delta middle Deweyville terrace

North Colorado- Depth (m) -30 bayhead delta stage 2 Deweyville terrace 5Galveston km Bay Dip Section (Trinity Valley) Matagorda Brazos fluvial D stage 1 100 km -40 Colorado-Delta offshore N panel c section S Bay -2 stage 3 Matagorda Brazos incised D Galveston Bay Dip Section (Trinity Valley) Bay Delta offshore valleys N panel c section S incised -2 stage 3 -6 stage 2 valleys stage 3 -6 stage 2 -10 stage 3 -10 Gulf of Mexico -14 Gulf of Mexico -14 -18 -18 stage 2 stage 2 Depth (m) Depth (m) -22 -22 Broadly same dimensions (Fig. A), same sea--26 10 km • stage 1 -26 10 km level history but different sediment rates -30 stage 1 -30 BR valley: exclusively amalgamated fluvialE F • 7 Brazos Valley 7 Trinity Valley deposits (Fig B) E Accommodation FAccommodation 6 7 Sediment supply 6 Brazos Valley 7 Sediment supply Trinity Valley Accommodation

/yr) Accommodation /yr) 3 TR valley: fluvial facies overlain by a distinct5 6 3 5 Sediment supply

m 6 Sediment supply m stage 3: auto-advance 6

• 6

4 /yr) flooding surface, then progradation bayhead 4 /yr) 3 5 3 5 m delta litho-facies, and then estuarine basin3 3 m stage 3: auto-advance 6 4 6 Rate (10

Rate (10 4 mud facies, followed by progradational2 2 bayhead delta in proximal areas of Trinity Bay1 3 1 3 5 10 15 20 25 5 10 15 20 25 Rate (10 and flood-tidal delta unit in the distal portions0 2 0 Rate (10 0 20 40 60 80 100 0 20 40 602 80 100 (Figs. C and D) Base level (m) Time (ky) 1 1 5 10 15 20 25 5 10 15 20 25 The model captures these differing first-order 0 0 0 20 40 60 80 100 0 20 40 60 80 100 stratigraphic patterns (Figs. E and F) Base level (m) Time (ky) Base level (m) Time (ky)