Deposition of Franciscan Complex cherts along the

paleoequator and accretion to the American margin at tropical

paleolatitudes

By Hagstrum, J. and Murchey, B.

Presented by Megan Simpson

Radiolarian Chert• In Western North America, basalt-chert

sequences of Mesozoic age

• Chert attributes• silica-rich, contains radiolarian skeletons• rhythmically bedded chert/shale• contain hematite with stable remanent magnetizations• thin sequences (<80m) represent 10-100 my

• Paleoceanographic origins within Pacific Basin are uncertain

Radiolarians from Franciscan Complex

(0.5-1.5mm silica shells)

Depositional Environment

• Early views felt cherts were deposited far from continental margins in the deep ocean– But deep-sea drilling failed to find thick deposits

• Alternative environments (need high radiolarian production):

• marginal basins• E-W trending zones near equator & higher (>45°)• N-S trending zones along western continental margin

Theory

• Authors propose FC chert of N. Cal originated at spreading center near the equator in Early Jurassic, followed low latitude trajectory toward continental margin before Late Cretaceous

• Geochemical studies show:– spreading ridge attributes at base– ocean basin/cont.margin at top

Paleomagnetic Investigations

• Can potentially provide paleolatitudes of deposition

• Previous studies have not succeeded• Only small percentage of rocks from Japan and

Mexico red chert show primary components of magnetization:– indicated deposition at equatorial paleolat.

• This study of No. Cal. FC was undertaken to enhance theory

Geologic Setting• Franciscan Complex is part of accretionary

wedge of ancient convergent plate margin (Late Jurassic/early Tertiary)

• Sheared argillite, graywacke, fragments of basalt, chert, limestone

• Undergone subduction-related metamorphism • Divided into 3 fault-bounded belts

– Coastal, Central, Eastern

Accretionary Wedge

Location map

Sample locations

• Chert collected from roadcut exposures

• The Geysers, Mt. Umunhum, Marin Headlands (Alexander Ave, McCollough Ave, Bonita Cove)

• Samples taken as oriented hand samples, red chert from base of sequences (thicker and less altered)

Sample Details

• Marin Headlands Block– 82 m of chert exposed– 7 radiolarian assemblages were identified– Age: late Pliensbachian to late Albian– Zones MH-1 through MH-7

• The Geysers– 67m (MH-2 through MH-7)

• Mt. Umunhum– ~50m (MH-1)

Sections of chert showing sampling positions (dots), black-normal polarity, white-reversed polarity. MH-1 L.Pliensbachian-mid Toarcian, MH-2 mid Toarcian -Aalenian, MH-3 Bajocian, MH-4 Bathonian - Callovian, MH-5 L. Tithonian - Hauterivian, MH-6 Hauterivian-Albian, MH-7 L. Albian-e.Cenomanian.

Magnetism Measurements

•Made using a cryogenic magnetometer

•Hematite is predominant magnetic mineral

•Specimens contain 3 components of remanent magnetization

(A) removed 300ºC, probably recently acquired thermoviscous magnetization

(B) removed 300-630ºC, interpreted as overprint (remagnetization) of normal polarity

(C) removed ~560-680ºC, magnetization predates structural deformation, shows both normal and reversed polarity

Determined Paleolatitudes

Paleoceanographic Model

• Paleolatitudes calculated from primary paleomagnetic direction indicate deposition near paleoequator during Pliensbachian - Bajocian– MH-1 (1º ± 2º)

– MH-2 (0º ± 2º)

– MH-3 (2º ± 4º)

• Represent about 25 million years

• Depositional rate 2.5 m/my for lower part of chert section

Model continued...• Modern Pacific equator has greatest diversity of

radiolarians due to stable environmental conditions

• Correlates to maximum species diversity in MH-1, 2, 3

• MH-4 through MH-7, species diversity decreases– Corresponds to decrease in rate of deposition, thickness

of beds

• Implies movement away from equatorial productivity

Initial Deposition

• Geochemical data for chert/shale indicate change upsection from pelagic depositional environment to terrigenous

• Lower rocks enriched in Fe, Mn imply deposition within zone of hydrothermal activity (near spreading center)

Transport• Paleomagnetic, biostratigraphic and

geochemical evidence for these cherts suggest initial deposit on mid-ocean ridge near equator

• Then transported toward American margin first along paleoequator, then away from it

• Turbidites overlying chert indicate arrival at American margin in Cenomanian time (95 ma)

Plate Reconstruction Model

Plots of true paleolatitude vs. time

Conversion from fixed NA coordinates to geographic using apparent polar wander path (APWP)

Thick curves show changes in paleolat at each docking pt

Plate Motion• Cherts now exposed in No. Cal (~38°N) implies

northward movement of ~4500 km (av. of 5cm/yr)

• Models indicate:– oblique convergence of Farallon, Kula to NA (mid

Cretaceous to Neogene)– transform motion between Pacific and NA (Neogene to

present)

• Uncertain if Kula or Farallon plate adjacent to margin

Transport of Franciscan Complex to American margin

Conclusion

• Paleomagnetic data for Mesozoic chert imply origins within equatorial zone (high biologic productivity)

• Unusual plate movements allowed for deposition of cherts along equator for 35 my, then relative motion turned north

Franciscan Complex TodayMarin Headlands

Franciscan Cherts

Serpentinite, red chert, sandstone