G. Della Porta, O. Merino-Tomé, J.A.M. Kenter and K. Verwer (2009) - Lower Jurassic microbial-sponge mounds and coral-sponge reefs in the Djebel Bou Dahar ramp to high-relief platform (High Atlas, Morocco)

1. LOWER JURASSIC AUTOMICRITE
-SPONGE MOUNDS AND CORAL-SPONGE REEFS
IN THE DJEBEL BOU DAHAR RAMP TO HIGH-RELIEF
PLATFORM (HIGH ATLAS, MOROCCO)
G. Della Porta*, O. Merino-Tomé**, J.A.M. Kenter*** & K. Verwer****
*Dipartimento Scienze della Terra, University of Milan, Italy
**IGME, Leon, Spain
***Chevron ETC, San Ramon, CA, USA
****RSMAS, Miami, FL, USA

2. Sinemurian to Pliensbachian deposystem evolution
Modified after Merino Tome’ et al. (2012, Sedimentology)
Sinemurian (Units I-III): low-relief ramp
Pliensbachian (Units IV-VI): high-relief steep slope

3. Sinemurian to Pliensbachian automicrite facies
Modified after Merino Tome’ et al. (2012, Sedimentology), Verwer et al. (2009, JSR)
Pliensbachian (Units IV-VI): upper-slope reef
Upper Sinemurian (Unit III): mounds
§ Pliensbachian upper
slope and margin
automicrite-coral-sponge
boundstone reef (high-
relief)
§ Upper Sinemurian
automicrite-sponge
mounds (ramp)

4. § How do the DBD mounds and reefs fit with the Jurassic
reef patterns?
§ What are the characteristics and possible origin of
automicrite (biologically induced precipitation,
microbially mediated/influenced or
organomineralisation)?

5. Jurassic reef patterns
From Leinfelder et al. (2002, SEPM Spec. Publ. 72)
§ Increasing reef diversity and frequency
§ Three Jurassic reef growth peaks (transgressive episodes)
1 Sponge mounds, coral reefs, Lithiotis reefs.
2 Coral-stromatoporoid reefs, siliceous sponge reefs.
3 Coral reefs, siliceous sponge-microbialite mud mounds,
coral-stromatoporoid reefs, coral-microbial reefs.
§ Recovery post T/J
extinction; first reef
domain Morocco
§ Synsedimentary
extensional
tectonics, Pangaea
breakup
§ Corals/reefs: oligo-
to mesotrophic
corals
bivalves
Siliceous
sponges
corals
microbial
Siliceous
sponges
1
2
3

6. Jurassic reef types
From Leinfelder et al. (2002, SEPM Spec. Publ. 72)
Siliceous sponge mounds: mid to outer
ramp; mostly Early Jurassic.
Bivalve (lithiotid) reefs: lagoon and margins
main factory in Pliensbachian (Scheibner &
Reijmer 1999; Wilmsen and Neuweiler
2008)
Microbialites typical of (upper) Jurassic
reefs, as major reef stabilizers.
Coral microbial debris reefs limited by
abrasion/resedimentation;
§ at sedimentation hiatuses and steep
bypass slopes.
§ mostly late Jurassic due to steepened
margins produced by rifting
Other Moroccan Pliensbachian reefs no
automicrite (cf. Wilmsen and Neuweiler
2008)

7. Mounds:
§ Up to 15 m-thick, 10’s wide, within by skeletal coated grain packstone.
§ Siliceous demosponges (lithistid and non) and hexactinellids (Hexactinosa Lyssacinosa),
echinoderms, brachiopods, bryozoans, bivalves, forams, Terebella and Radiomura cautica.
§ Automicrite with homogeneous, clotted peloidal and laminated fabrics, gravity defying
structures and isolating stromatactis-like cavities filled by radial fibrous cement.
Upper Sinemurian siliceous sponge-automicrite mounds
(middle ramp)
From Della Porta et al. (2013, SEPM Spec. Publ.)

8. Upper Sinemurian siliceous sponge-automicrite mounds
Hexactinosa, Terebella, aphanitic automicrite Lithistid demosponges and clotted peloidal
micrite
Hexactinosa, Lithistid, coated grain packstone Laminated peloidal micrite
From Della Porta et al. (2013, SEPM Spec. Publ.)

9. Pliensbachian coral-sponge-automicrite boundstone (high-relief slope)
§ slopes: up to 450-600 m relief, 25-30° steep
§ sponge -automicrite boundstone: in 1-4 m thick, 60-140 m deep below the platform break,
alternating with breccias and grain-rudstone platform-derived resediments;
§ coral-sponge-automicrite boundstone: 1-12 m thick, between platform-break down to
depths of 70-100 m; surrounded by platform margin grainstone
§ Coral-sponge-automicrite in situ factory + resedimented deposits > 50% of the slope
From Della Porta et al. (2013, SEPM Spec. Publ.)

10. Pliensbachian siliceous sponge-automicrite boundstone on upper
slope
§ hexactinosa and lyssacinosa
(hexactinellids) + lithistid and non
demosponges
§ stromatactis-like cavities infilled by
radial and radiaxial fibrous cement
§ aphanitic to laminated peloidal
micrite
From Della Porta et al. (2013, SEPM Spec. Publ.)

11. Pliensbachian coral-sponge-automicrite boundstone on uppermost
slope
§ m-scale colonies of phaceloid corals
§ micrite crusts and radial fibrous cement
bind skeletal framework
§ stromatoporoids, chaetetids, demosponges,
Bacinella ordinata, Baccanella floriformis,
Tubiphytes, Radiomura cautica
§ No hexactinellids
From Della Porta et al. (2013, SEPM Spec. Publ.)

12. SEM: presence of organic structures, EPS?
§ possible evidence of preserved EPS with sub-polygonal structure
§ organic substrates incorporated in the carbonate
§ globular structures still preserved on the diffused organic mass
§ based on the comparison with modern carbonate stromatolites (Vasconcelos pers. com.; cf.
Spadafora et al. 2009)

13. Stable isotopes: automicrite as marine precipitates
Upper Sinemurian mounds:
§ Isotopic values of brachiopods, RF
cement and automicrite
≠ burial cement
= literature marine values (E. J.)
Pliensbachian slope boundstone:
§ Isotopic values of brachiopods,
RF cement
= literature marine values (E. J.)
§ Automicrite
partly = marine values
partly = O and C depletion due to
burial diagenesis
§ precipitated in equilibrium with
seawater
§ non enzymatic fractionation
From Della Porta et al. (2015, Chemical Geology)

14. Conclusions
þ Upper Sinemurian siliceous sponge mounds share similarities for facies, biota, automicrite
fabrics and depositional setting with other Lower Jurassic mounds.
þ Siliceous sponge mounds grew in middle to outer ramp, aphotic settings probably
controlled OMZ and nutrients (cf. Neuweiler et al. 2001) and were buried by sediment
input of coated grain grainstone.
þ Pliensbachian tectonically-controlled, high-relief slopes and margins are dominated by
coral-automicrite-sponge reefs substituted downslope by lenses of siliceous sponge-
automicrite boundstone with water depth zonation of biota and automicrite fabrics.
þ DBD upper slope automicrite boundstone shares similarities with Upper Jurassic coral-
microbial reefs (cf. Leinfelder et al. 2002 and reference therein) rather than other
Pliensbachian automicrite-free platforms in Morocco (Wilmsen and Neuweiler 2008).
þ Automicrite precipitation persisted from Sinemurian to Pliensbachian because of specific
local environmental conditions (supersaturation, nutrients) “adjusting” to the tectonically-
controlled evolution of geometry from ramp to high-relief and anticipating the Upper
Jurassic coral-microbial reefs.

15. Conclusions
ý The Pliensbachian DBD margin and slope carbonate factory does not receive the major
contribution by Lithiotid banks and lagoon facies (Scheibner and Reijmer, 1999; Wilmsen
and Neuweiler 2008) and it has automicrite precipitation also at the Pliensbachian.
ý Processes of automicrite precipitation are undeterminable but products are typical of
biologically induced/influenced precipitates and microbial carbonates.
ý Isotopic signature in equilibrium with marine waters; no enzymatic fractionation,
organomineralisation?
ý Organic substrate remnants (EPS?) are present but not discriminant for biologically
induced /influenced or miocrobially mediated precipitation.
ý Further geochemical analysis necessary: trace elements and REE (in progress)
Thank you