Ellsworth-Whitmore Mountains Block Research Articles

The Gondwana Plate margin in the Weddell Sea sector: Zircon geochronology of Upper Paleozoic (mainly Permian) strata from the Ellsworth Mountains and eastern Ellsworth Land, Antarctica

Zircon geochronology has been applied to upper Paleozoic (mainly Permian) strata from the Ellsworth Mountains and from enigmatic outcrops in eastern Ellsworth Land to aid in constraining their locations in the Weddell Sea sector of the Gondwana plate margin. Detrital zircons from two Polarstar Formation (Ellsworth Mountains) sandstones yielded maximum depositional ages of ~270Ma and ~260Ma. Detrital zircons from Permian beds at Erehwon Nunatak and beds of uncertain age at FitzGerald Bluffs, both in eastern Ellsworth Land, gave maximum depositional ages of ~264Ma and ~525Ma respectively. Two Polarstar Formation tuffs gave ages of 258.0±2.1Ma and 262.6±2.2Ma. The Polarstar Formation probably spans the entire Permian. The quartzite at FitzGerald Bluffs is comparable in composition and detrital zircon signature to Devonian sandstones in both the Ellsworth and Transantarctic Mountains. Evaluation of the Polarstar Formation with respect to Permian strata in the southern Karoo and Falkland Islands suggests deposition in a separate and distinct basin, which was related to that in the Transantarctic Mountains. A separate depositional basin forms an additional constraint on the location of the Ellsworth-Whitmore block adjacent to the Coats Land/Pensacola Mountains region. The eastern Ellsworth Land outcrops are inferred to document a small continental block located proximal to the Ellsworth-Whitmore Mountains block during Gondwana time.

The A-type Pirrit Hills Granite, West Antarctica: an example of magmatism associated with the Mesozoic break-up of the Gondwana supercontinent

The Mesozoic geology of West Antarctica is largely related with the break-up of the Gondwana supercontinent and offers a good example for understanding magmatism associated with the continental break-up process. West Antarctica can be divided into five crustal blocks with relatively thin crust. The blocks are separated by deep rift zones and have moved during the Mesozoic break-up of Gondwana. The Pirrit Hills granite occurs as an isolated pluton in the Ellsworth-Whitmore Mountains block, which is the center of five blocks in the present configuration. The granite consists of quartz, perthitic alkali feldspar, and plagioclase with minor amounts of interstitial biotite and muscovite. The granite is a highly homogeneous, strongly fractionated, and mildly peraluminous granite and belongs to A-type granites with A2-type characteristics, suggesting its generation in an anorogenic environment. The strong enrichment of HREE and significant negative Eu anomalies suggest that the granitic magma was produced by a small degree of partial melting of a garnet granulitic source in the unusually hot lower crust. A weighted mean 206Pb/238U age of zircons is 164.5 ± 2.3 Ma (MSWD = 1.3), which is 8 to 9 Mys younger than a former Rb-Sr whole rock age (173 ± 3 Ma), and corresponds to the first rifting stage of the break-up of Gondwana (at 165 Ma). We suggest this age to be the emplacement age of the Pirrit Hills granite. The A-type Pirrit Hills granite was emplaced in the Middle Jurassic accompanying crustal thinning due to the break-up of Gondwana.

Geochemical and isotopic constraints on the tectonic and crustal evolution of the Shackleton Range, East Antarctica, and correlation with other Gondwana crustal segments

Three distinct, spatially separated crustal terranes have been recognised in the Shackleton Range, East Antarctica: the Southern, Eastern and Northern Terranes. First insights into the origin and evolution of critical units from these terranes could be gained from new lithogeochemical and isotope (Rb–Sr, Sm–Nd, Pb–Pb whole rock and Lu–Hf zircon) data. Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710–1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica. The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600–1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ∼600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa. Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510–500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana.

Central Antarctic provenance of Permian sandstones in Dronning Maud Land and the Karoo Basin: Integration of U–Pb and TDM ages and host-rock affinity from detrital zircons

In conjugate SE Africa and Antarctica, Early Permian sandstones of the Swartrant Formation of the Ellisras Basin, Vryheid Formation of the Karoo Basin, and Amelang Plateau Formation of Dronning Maud Land (DML) were deposited after Gondwanan glaciation on a westward paleoslope. We analysed detrital zircons for U–Pb ages by a laser ablation microprobe-inductively coupled plasma mass spectrometer (LAM-ICPMS) and attached age significance only to clusters of three or more overlapping analyses. We analysed Hf-isotope compositions by a multi-collector spectrometer (LAM-MC-ICPMS) and trace elements by electron microprobe (EMP) and ICPMS. These analyses indicate the rock type and source (whether crustal or juvenile mantle) of the host magma, and a “crustal” model age ( T DM C). The integrated analysis gives a more distinctive, and more easily interpreted, picture of crustal evolution in the provenance area than age data alone. Zircons from the Ellisras Basin are aged 2700–2540 Ma with minor populations about 2815 Ma and 2040 Ma, which correspond with the ages of the upslope parts of the proximal Kaapvaal Craton and Limpopo Belt. Mafic rock is the dominant host rock, and it reflects the Archean granite–greenstone terrane of the Kaapvaal Craton. The three Karoo Basin samples and the two DML samples have zircons with these common properties: (1) 1160–880 Ma, host magma mafic granitoid (< 65% SiO 2) derived from juvenile depleted mantle sources ( ε Hf positive) at 1.65 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga; (2) 760 to 480 Ma, host magma granitoid and low-heavy rare earth element rock (?alkaline rock–carbonatite), derived from mixed crustal and juvenile depleted mantle sources ( ε Hf positive and negative) at 1.50 Ga and 1.35 Ga, with T DM C of 2.0–0.9 Ga. Together with similar detrital zircons in Triassic sandstone of SE Australia, these properties reflect those in upslope central Antarctica, indicating a provenance of ∼ 1000 Ma (Grenville) cratons embedded in 700–500 Ma (Pan-Gondwanaland) fold belts. Detrital zircons in Cambrian sediments of the Ellsworth–Whitmore Mountains block and Cambrian metasediments of the Welch Mountains with comparable properties suggest that the central Antarctic provenance operated also in the ∼ 500 Ma Cambrian.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica

Research Article| March 01, 2007 Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica M.J. Flowerdew; M.J. Flowerdew 1Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar I.L. Millar; I.L. Millar 1Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar M.L. Curtis; M.L. Curtis 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar A.P.M. Vaughan; A.P.M. Vaughan 2British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 0ET, UK Search for other works by this author on: GSW Google Scholar M.S.A. Horstwood; M.S.A. Horstwood 3Natural Environmental Research Council (NERC) Isotope Geosciences Laboratory, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK Search for other works by this author on: GSW Google Scholar M.J. Whitehouse; M.J. Whitehouse 4Swedish Museum of Natural History, Box 50007, SE-104 05, Stockholm, Sweden Search for other works by this author on: GSW Google Scholar C.M. Fanning C.M. Fanning 5Research School of Earth Sciences, Australian National University, Mills Roads, Canberra ACT 0200, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (2007) 119 (3-4): 275–288. https://doi.org/10.1130/B25891.1 Article history received: 25 Aug 2005 rev-recd: 21 Jun 2006 accepted: 11 Sep 2006 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation M.J. Flowerdew, I.L. Millar, M.L. Curtis, A.P.M. Vaughan, M.S.A. Horstwood, M.J. Whitehouse, C.M. Fanning; Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica. GSA Bulletin 2007;; 119 (3-4): 275–288. doi: https://doi.org/10.1130/B25891.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb detrital zircon geochronology from the upper Cambrian to Devonian part of the Ellsworth Mountains succession, Antarctica, yields dominant late Mesoproterozoic and late Neoproterozoic–Cambrian age populations that are consistent with a provenance from within Gondwana. Hf isotope compositions reveal a source predominantly within west Gondwana and identify a change in provenance up-stratigraphy that coincides with the change of sedimentation setting from active rift to passive margin, which has been independently determined by stratigraphic, structural, and geochemical arguments. For the Late Cambrian Frasier Ridge Formation, late Mesoproterozoic grains have positive ϵHf values, suggesting derivation from juvenile crust, and late Neoproterozoic–Cambrian grains have ϵHf values greater than −5, consistent with remelting of similar juvenile late Mesoproterozoic crust during the Pan African–Ross orogenies. Provenance during rifting was from proximal sources from within west Gondwana, most likely, southernmost Africa and basement to the Ellsworth-Whit-more Mountains block. At higher stratigraphic levels where deposition occurred along a passive margin, in the early Ordovician Mount Twiss Member and middle Devonian Mount Wyatt Earp Formation, late Neo-proterozoic–Cambrian grains have ϵHf values less than −5; this means that early Meso-proterozoic–Archean crust was remelted to generate these zircons. Provenance was from a more expansive source region within west Gondwana, and probably included the Kaapvaal and Congo cratons of south and west Africa. Isolated outcrops of sedimentary rock of uncertain age at Mount Woollard and the Whitmore Mountains have detrital zircon signatures similar to the Frasier Ridge Formation, suggesting correlation with these Late Cambrian deposits. Sedimentary rock from the Stewart Hills contains some late Mesoproterozoic grains with lower ϵHf values than the previously mentioned samples. This suggests that the Stewart Hills sample has a provenance from within east Gondwana and was possibly deposited on the East Antarctic craton prior to the Ross orogeny and is not part of the displaced Ellsworth-Whitmore Mountains crustal block. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

A window on West Antarctic crustal boundaries: the junction between the Antarctic Peninsula, the Filchner Block, and Weddell Sea oceanic lithosphere

A new airborne magnetic survey of the southeastern Antarctic Peninsula and adjacent Weddell Sea embayment (WSE) region suggests a continuity of geological structure between the eastern Antarctic Peninsula and the attenuated continental crust of the Filchner Block. This has implications for the reconstructed position of the Ellsworth–Whitmore Mountains block in Gondwana, which is currently uncertain. Palaeomagnetic data indicate that it has migrated from a Palaeozoic position between South Africa and Coats Land to its current position as a microplate embedded in central West Antarctica. The most obvious route for migration is between the Antarctic Peninsula and the Weddell Sea embayment. Evidence that geological structures are continuous across the boundary places constraints on the timing and pathway of migration. Magnetic textures suggest the presence of shallow features extending from the Beaumont Glacier Zone (BGZ) in the west for at least 200 km into the Weddell Sea embayment. These data suggest that the Eastern Domain of the Antarctic Peninsula and the stretched continental crust of the Filchner Block share a common recent, probably post-Early Jurassic, history. However, examination of deep anomalies indicates differences in the magnetic characteristics of the two blocks. The boundary may mark either the edge of extended continental crust, or a discontinuity between two, once separated, blocks. This discontinuity, or pre-Late Jurassic Antarctic Peninsula terrane boundaries to the west, may have allowed the passage of the Ellsworth–Whitmore Mountains block to its present location.

Evidence for Cambrian deformation in the Ellsworth-Whitmore Mountains terrane, Antarctica: Stratigraphic and tectonic implications

Research Article| January 01, 1998 Evidence for Cambrian deformation in the Ellsworth-Whitmore Mountains terrane, Antarctica: Stratigraphic and tectonic implications Ernest M. Duebendorfer; Ernest M. Duebendorfer 1Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Margaret N. Rees Margaret N. Rees 2Department of Geoscience, University of Nevada, Las Vegas, Nevada 89154 Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (1): 55–58. https://doi.org/10.1130/0091-7613(1998)026<0055:EFCDIT>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Ernest M. Duebendorfer, Margaret N. Rees; Evidence for Cambrian deformation in the Ellsworth-Whitmore Mountains terrane, Antarctica: Stratigraphic and tectonic implications. Geology 1998;; 26 (1): 55–58. doi: https://doi.org/10.1130/0091-7613(1998)026<0055:EFCDIT>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The Ellsworth-Whitmore Mountains terrane is a large geologically and geophysically defined crustal block that lies between the Transantarctic Mountains and West Antarctica. The Cambrian position of the terrane is controversial, with many workers placing it between East Antarctica and southern Africa and distant from Cambrian orogenic belts. We present structural and stratigraphic evidence for Cambrian deformation in the Heritage Range, Ellsworth Mountains. From our revised stratigraphy and structural history of the Heritage Range, we propose that the Ellsworth-Whitmore Mountains block was located within the belt of Pan-African deformation, within the Late Cambrian continental arc, and was part of a collage of allochthonous terranes that included the Queen Maud terrane and probably the Bowers terrane of Antarctica. These terranes were situated outboard of Coats Land in the Cambrian and were subsequently translated and accreted to East Antarctica, probably during early Paleozoic time. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Evidence for Cambrian deformation in the Ellsworth-Whitmore Mountains terrane, Antarctica: Stratigraphic and tectonic implications: Comment and Reply

The Ellsworth-Whitmore Mountains terrane is a large geologically and geophysically defined crustal block that lies between the Transantarctic Mountains and West Antarctica. The Cambrian position of the terrane is controversial, with many workers placing it between East Antarctica and southern Africa and distant from Cambrian orogenic belts. We present structural and stratigraphic evidence for Cambrian deformation in the Heritage Range, Ellsworth Mountains. From our revised stratigraphy and structural history of the Heritage Range, we propose that the Ellsworth-Whitmore Mountains block was located within the belt of Pan-African deformation, within the Late Cambrian continental arc, and was part of a collage of allochthonous terranes that included the Queen Maud terrane and probably the Bowers terrane of Antarctica. These terranes were situated outboard of Coats Land in the Cambrian and were subsequently translated and accreted to East Antarctica, probably during early Paleozoic time.

New paleomagnetic data from the Antarctic Peninsula and their tectonic implications

New paleomagnetic data presented here further constrain the relative motion of the Antarctic Peninsula relative to East Antarctica during the Mesozoic development of the southern ocean basins. The Antarctic Peninsula (AP) is one of four crustal blocks that define West Antarctica, the others being the Ellsworth‐Whitmore Mountains (EWM), Thurston Island‐Eights Coast (TI), and Marie Byrd Land (MBL). A Jurassic pole (∼155Ma) (124°E, 64°S, A95=7.1°, N=10 VGPs) was obtained from the AP block which suggests that the AP block rotated clockwise between ∼175 and ∼155 Ma due to significant early opening in the Weddell Sea basin. A new Early Cretaceous paleomagnetic pole (182°E, 74S, A95=5.9°, N=6 site mean VGPs) indicates that the AP block was in or near to its present‐day position with respect to East Antarctica by ∼130 Ma. Between ∼155 and 130 Ma, counterclockwise rotation of the AP‐TI blocks, together with the southward motion of East Gondwanaland, probably resulted in subduction of Weddell Sea ocean floor beneath the southern AP block and initiated the Palmer Land deformational event. A ∼130 Ma pole from the TI block requires clockwise rotation of the TI and possibly the EWM blocks between 130 and ∼110 Ma producing sinsistral strike‐slip motion between the EWM block and East Antarctica and dextral transpressional motion between the TI‐EWM blocks and the AP block. New AP block ∼110 Ma and ∼85 Ma poles from this study (199°E, 74°S, A95=6.9°, N=13 VGPs; 152°E, 86°S, A95 = 7.5°, N=6 VGPs, respectively) are similar to equivalent age poles from East Antarctica and suggest little or no relative motion between the Antarctic Peninsula and East Antarctica. Northern and southern Antarctic Peninsula mid‐Cretaceous poles are very much alike suggesting that the “S” shape of the Antarctic Peninsula is not due to oroclinal bending since ∼110 Ma.

New paleomagnetic data from Thurston Island: Implications for the tectonics of west Antarctica and Weddell Sea opening

Paleomagnetic data from three West Antarctic crustal blocks (Antarctic Peninsula (AP), Thurston Island‐Eights Coast (TI), and the Ellsworth‐Whitmore Mountains (EWM) indicate that there has been motion between the individual blocks and motion relative to East Antarctica during the Mesozoic. A Triassic paleomagnetic pole from the TI block (116°E, 61°S, A95 = 19.4°, N = 3 VGPs) appears to indicate that the block has rotated ∼90° relative to East Antarctica between 230 Ma and 110 Ma. Our previously reported Middle Jurassic paleomagnetic pole from the EWM block indicates that a 90° rotation relative to East Antarctica occurred sometime between the Cambrian and 175 Ma. We believe that the 90° counterclockwise EWM rotation occurred between ∼220 Ma and 175 Ma related to the development of post‐Gondwanide Orogeny shear zones. The motion of the AP, TI, and EWM blocks appears to be linked during the mid‐to late Mesozoic to three major events in the evolution of the southern ocean basins. Opening in the Mozambique‐Somali‐Weddell Sea basins may have produced major counterclockwise rotation of the TI block with respect to East Antarctica between the Jurassic and Early Cretaceous based on new Late Jurassic (145°E, 64.5°S, A95 = 7°,N = 5 VGPs) poles. We believe that the TI rotation, as well as deformation in the southern AP block, was caused by collision and shearing of the EWM block against the other two as the EWM block moved southward with East Antarctica. An Early Cretaceous paleomagnetic pole (232°E, 49°S, A95 = 7.9°, N = 5 VGPs) from the TI block requires that between the Early and mid‐Cretaceous there was clockwise rotation, with respect to East Antarctica, of the AP‐TI‐EWM blocks (an entity we call Weddellia). A change in the opening history of the Weddell Sea basin caused by initiation of spreading in the South Atlantic ocean basin at ∼130 Ma probably started Weddellia's clockwise rotation. Two new ∼110 and ∼90 Ma poles from the TI block (210°E, 73°S, A95 = 7.6°,N = 7 VGPs and 161°E, 81°S, A95 = 3.9°,N = 18 VGPs, respectively) are similar to equivalent age poles from the AP block and East Antarctica and indicate that Weddellia was at or near its present‐day position with respect to East Antarctica by ∼110 Ma. This corresponds to a time of major plate reorganization in the South Atlantic and southeast Indian Oceans. Based on both the new TI paleomagnetic data and previously reported data from Marie Byrd Land (MBL), dextral shearing would be expected to have occurred between MBL and Weddellia since the mid‐Cretaceous. Pine Island Bay, the area between the TI and MBL blocks, marks a fundamental and complex tectonic boundary in West Antarctica that we propose has largely been a zone of transcurrent shearing.

Mesozoic evolution of West Antarctica and the Weddell Sea Basin: new paleomagnetic constraints

Paleomagnetic data from the Antarctic Peninsula and our recent results from the Ellsworth-Whitmore Mountains block suggest that since the Middle Jurassic these two West Antarctic blocks have undergone little relative movement and together have rotated relative to the East Antarctic craton. New data from Lower Cretaceous rocks from the Thurston Island region of West Antarctica suggest that on the basis of paleomagnetic constraints, the Antarctic Peninsula, Ellsworth-Whitmore Mountains and Thurston Island blocks define a single entity which we call Weddellia; some motion between these blocks is possible within the limits of the paleomagnetic data. Between the Middle Jurassic and Early Cretaceous, Weddellia remained attached to West Gondwanaland while East Antarctica moved southward (dextrally) relative to Weddellia. From the Early Cretaceous to mid-Cretaceous, Weddellia rotated clockwise 30° and moved sinistrally approximately 2500 km relative to East Antarctica, to its present-day position. We suggest the Early to mid-Cretaceous to be the time of the main if not initial opening of the Weddell Sea.