Geochemical and Chronostratigraphic Record of Cenomanian-Tu - PowerPoint Presentation

Slide1

Geochemical and Chronostratigraphic Record of Cenomanian-Turonian Eagle Ford, South Texas, USA

Master’s Saturday December, 2016

Michael NietoSlide2

Goal and Objectives

Goal: to collect

20

bentonite ash beds from 3 subsurface cores in Atascosa County, and analyze them for U-

Pb

absolute age dates.

Objectives

:

Use age dates to provide a local chronostratigraphic frameworks to understand timing of Eagle Ford facies distribution.

Estimate Eagle Ford accumulation rates in South Texas.

Integrate chronostratigraphic framework with regional

biostratigraphic

data and previous U-

Pb

geochronology.

Identify likely source of volcanism for Eagle Ford bentonites.Slide3

Geologic Background

Modified from Denne, 2014

Major Structural Elements:

San Marcos Arch

Maverick Basin

East Texas Basin/Woodbine

Cretaceous Reef Margins Slide4

Separation Methodology

Presence of abundant clay minerals makes mineral separation difficult in ash beds.

Prior to traditional mineral separation, samples were put through sonication system.

This rids samples of their clay fractions and increases retrievable zircon population by keeping clays from flocculating.

After sonication, samples were processed using traditional mineral separation techniques (LMT, Frantz, MEI)

SONICATIONSlide5

Analysis Methodology

Small population of zircons were imaged using CL

Picture above demonstrates radial zoning and presence of cores, which must be acknowledged prior to LA-ICP-MS

Depth profiling was used to help distinguish between multiple age domains and pick the youngest eruptive age, while also preserving the signature imparted by older cores.Slide6

A-3 U-

Pb

Age Dates

Base of Eagle Ford – 95.9 +/- 0.5Ma (early Cenomanian).

Cores show 103-108 Ma.

A-3.6 ash – 95.0 +/-0.4Ma.

Cores in this sample show similar dates (104-109).

LEF-UEF lithologic transition confined between 94.6-94.2 Ma (late Cenomanian).

Eagle Ford and Austin contact 89.1 +/- 0.7 Ma, within error of Turonian-Coniacian stage boundary (89.8 Ma).Slide7

A-4 U-

Pb

Age Dates

Base of Eagle Ford – 96.0 +/- 0.6 Ma (early Cenomanian), similar to A-3 core.

Cores for this basal samples – 104-109Ma, similar to LEF cores in A-3 core.

A-4.6 – 93.0 +/- 0.6Ma, slightly younger than expected (1 Ma).

Mode – 94Ma

Unequal distribution towards young ages <94Ma.

Incorporation of

metamict

cores and/or

Pb

loss domains.

A-4.4 – 94.3 +/- 0.5 Ma, consistent with A-3 core.

Austin-Eagle Ford contact constrained by 88.3 and 91.5 Ma, consistent with Turonian-Coniacian stage boundarySlide8

A-5 U-

Pb

Age Dates

Del Rio ash – 107.6 +/- 0.7 Ma. Albian

Basal Eagle Ford ash 106.1 +/- 0.6 Ma, too old

8” ash, not seen in surrounding cores

Sharp, angular contact. Faint laminations. Erosive?

Reworked?

A-5.4 large spread in zircon data, multi-modal and interpreted as reworked.

A-5.3 – 95.5 +/- 0.5 Ma, similar to A-3 and A-4 cores.

Austin-Eagle Ford contact – 87.2 +/-0.6 Ma. Turonian-Coniacian boundarySlide9

Regional Cross Section of Isotope EventsSlide10

Regional Cross Section U-

Pb

Age Dates

C-T boundary placed in UEF of A-1 based on Denne (2014)

biostrat

.

C-T boundary

Eldrett

et al. (2015) in UEF and dated 94.10 +/- 0.13 Ma.

Boundary inferred in current study based on U-

Pb

age dates.Slide11

Fingerprint Volcanic Rock Type

TAS diagrams susceptible to alteration and mobility of elements.

Zr

/TiO2 -

Nb

/Y diagram (Winchester & Floyd, 1977

) utilizes immobile elements to categorize rock type.

Intermediate to evolved (

Andestie

– Rhyolite), typical volcanic arc lavas.

Albian age cores, intermediate rock type, and studies showing activity during this time – volcanic arc off west coast of Mexico likely source of EF ashes (

Silver and Chappell,

1988).

Sedimentation rates --- LEF: 1.3-2.5cm/

ka

and UEF: 0.08-0.14cm/

ka

(complexed by hiatal surface in UEF)

A-3 core: circles, A-4 core: triangles, A-5 core: squares

Purple – Del Rio, Black – LEF, Grey – UEF, Blue – Austin Chalk. Slide12

Conclusions

Transition from Buda Limestone to Eagle Ford Shale represents a major Cretaceous transgression and deposition of Eagle Ford began during the early Cenomanian (96 Ma).

The

lithostratigraphic

boundary separating the lower from upper Eagle Ford marks a transition from anoxic to

suboxic

conditions. Transition occurred during the late Cenomanian and is dated around 94.6 Ma.

Transition from Eagle Ford to Austin Chalk represents a return to oxygenated bottom water and the contact between the two coincides closely with the Turonian-Coniacian stage boundary (89.1Ma). An erosive/hiatal surfaces is inferred in the upper Eagle Ford based on missing isotope events, large gap in age dates, thin stratigraphic section and extrapolation of nearby

biostratigraphic

datums

.

Placement of the Cenomanian-Turonian boundary is inferred, due to the suggested presence of missing time in upper Eagle Ford, and no direct biostratigraphic Chemical analysis of Eagle Ford ash beds reveal a more intermediate soure of volcanic rocks for this system. Coupled with the presence of Albian age cores, a likely volcanic source is identified as the magmatic arc in western Mexico (particularly Peninsular Ranges).

Sedimentation rates --- LEF: 1.3-2.5cm/

ka

and UEF: 0.08-0.14cm/

ka

(complexed by hiatal surface in UEF)Slide13

Acknowledgments

Thanks:

Harry Rowe

EOG Resources and Nestor Phillips

MSRL consortia members

Statoil

Danny and Lisa Stockli

Geochron Group: Doug, Kelley, Margo, DanBEG Crew: Evan Sivil, Nate Ivicic

, Brandon Williamson and Bill MolthenFellow grads: Chris Hendrix and Lauren Redmond